Semiconductor device

ABSTRACT

A semiconductor device including a capacitor with increased charge capacity and having a high aperture ratio and low power consumption is provided for a semiconductor device including a driver circuit. The semiconductor device includes a driver circuit which includes a first transistor including gate electrodes above and below a semiconductor film so as to overlap with the semiconductor film; a pixel which includes a second transistor including a semiconductor film; a capacitor which includes a dielectric film between a pair of electrodes in the pixel; and a capacitor line electrically connected to one of the pair of electrodes. In the semiconductor device, the gate electrode over the semiconductor film of the first transistor is electrically connected to the capacitor line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed in this specification and the like relates tosemiconductor devices.

2. Description of the Related Art

In recent years, flat panel displays such as liquid crystal displays(LCDs) have been widespread. In each of pixels provided in the rowdirection and the column direction in a display device such as a flatpanel display, a transistor serving as a switching element, a liquidcrystal element electrically connected to the transistor, and acapacitor connected to the liquid crystal element in parallel areprovided.

As a semiconductor material for forming a semiconductor film of thetransistor, a silicon semiconductor such as amorphous silicon orpolysilicon (polycrystalline silicon) is generally used.

Metal oxides having semiconductor characteristics (hereinafter referredto as oxide semiconductors) can be used for semiconductor films intransistors. For example, techniques for forming transistors includingzinc oxide or an In—Ga—Zn-based oxide semiconductor are disclosed (seePatent Documents 1 and 2).

REFERENCE

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

SUMMARY OF THE INVENTION

In a capacitor, a dielectric film is provided between a pair ofelectrodes at least one of which is formed, in many cases, using alight-blocking film partly serving as a gate electrode, a sourceelectrode, a drain electrode, or the like of a transistor.

As the capacitance value of a capacitor is increased, a period in whichthe alignment of liquid crystal molecules of a liquid crystal elementcan be kept constant in the state where an electric field is applied canbe made longer. When the period can be made longer in a display devicewhich displays a still image, the number of times of rewriting imagedata can be reduced, leading to a reduction in power consumption.

One of methods for increasing the charge capacity of a capacitor is toincrease the area occupied by the capacitor, specifically, to increasethe area of a region where a pair of electrodes is overlapped with eachother. However, when the area of a light-blocking conductive film isincreased to increase the area of a region where a pair of electrodes isoverlapped with each other, the aperture ratio of a pixel is lowered andthus display quality of an image is degraded.

In addition, a transistor included in a driver circuit of the displaydevice can be formed by utilizing a process for forming a transistorprovided in a pixel of the display device. The transistor included inthe driver circuit is required to operate at higher speed than thetransistor provided in the pixel, and the operation speed of thetransistor included in the driver circuit can be improved by applying atransistor in which gate electrodes are provided above and below asemiconductor film so as to overlap with the semiconductor film(hereinafter also referred to as a dual-gate transistor) to thetransistor. However, the application of the dual-gate transistorinvolves a structure which is configured to control the potential of thegate electrodes provided above and below the semiconductor film so as tooverlap with the semiconductor film (the structure including a wiring, aterminal, a power source, and the like), resulting not only in anincrease of the power consumption of the driver circuit but also in anincrease of the power consumption of the display device.

Thus, it is an object of one embodiment of the present invention toprovide a semiconductor device including a capacitor with increasedcharge capacity and having a high aperture ratio for a semiconductordevice including a driver circuit. In addition, it is another object ofone embodiment of the present invention to provide a semiconductordevice with low power consumption, which includes a capacitor withincreased charge capacity and having a high aperture ratio, for asemiconductor device including a driver circuit.

In view of the above description, according to one embodiment of thepresent invention, a semiconductor device includes a driver circuitwhich includes a first transistor including gate electrodes above andbelow a semiconductor film so as to overlap with the semiconductor film;a pixel which includes a second transistor including a semiconductorfilm; a capacitor which includes a dielectric film between a pair ofelectrodes in the pixel; and a capacitor line electrically connected toone of the pair of electrodes. In the semiconductor device, the gateelectrode over the semiconductor film of the first transistor iselectrically connected to the capacitor line.

More specifically, according to another embodiment of the presentinvention, a semiconductor device includes a driver circuit whichincludes a first transistor including gate electrodes above and below asemiconductor film so as to overlap with the semiconductor film; a pixelwhich includes a second transistor including a semiconductor film; acapacitor which includes a dielectric film between a pair of electrodesand a pixel electrode electrically connected to the second transistor,which are in the pixel; and a capacitor line electrically connected toone of the pair of electrodes. In the semiconductor device, the gateelectrode over the semiconductor film of the first transistor iselectrically connected to the capacitor line, the capacitor includes asemiconductor film on the same surface as the semiconductor film of thesecond transistor and the semiconductor film serves as the one of thepair of electrodes, the pixel electrode serves as the other of the pairof electrodes, and the dielectric film is an insulating film over thesemiconductor film of the second transistor.

In the capacitor, the one electrode is formed using thelight-transmitting semiconductor film of the second transistor, theother electrode is formed using the light-transmitting pixel electrodewhich is electrically connected to the second transistor, and thedielectric film is formed using the light-transmitting insulating filmover the light-transmitting semiconductor film of the second transistor.That is, the capacitor transmits light and thus can be formed large (ina large area) in a region except a portion where the transistor in apixel is formed. For this reason, the semiconductor device can havecharge capacity increased while the aperture ratio is improved. Thus,the semiconductor device can have an excellent display quality.

Further, as in the first transistor, the gate electrode provided overthe semiconductor film where a channel formation region is formed(hereinafter referred to as a back-gate electrode) and the capacitorline electrically connected to the one electrode of the capacitor areelectrically connected to each other; thus, the potential of theback-gate electrode can be controlled by controlling the potential ofthe capacitor line. That is, a structure which is configured to controlthe potential of the back-gate electrode can be omitted and the firsttransistor can be operated as a dual-gate transistor; accordingly, theoperation speed of the driver circuit can be increased. In other words,in the case where the first transistor is driven as the dual-gatetransistor, the mobility of the first transistor can be increased.Accordingly, a semiconductor device which can achieve both an increasein operation speed and a decrease in power consumption can be obtained.

Further, the light-transmitting semiconductor film can be formed usingan oxide semiconductor. This is because an oxide semiconductor has anenergy gap as wide as 3.0 eV or more and high visible-lighttransmissivity. In the description below, the light-transmittingsemiconductor film can be simply referred to as an oxide semiconductorfilm. Thus, the second transistor is a transistor including an oxidesemiconductor film, and the one electrode of the capacitor is formedusing the oxide semiconductor film.

A light-transmitting capacitor can be formed by utilizing the processfor forming the second transistor. The one electrode of the capacitorcan be formed by utilizing the process for forming the oxidesemiconductor film of the second transistor, the dielectric film of thecapacitor can be formed by utilizing the process for forming theinsulating film provided over the semiconductor film of the secondtransistor, and the other electrode of the capacitor can be formed byutilizing the process for forming the pixel electrode electricallyconnected to the second transistor.

The first transistor included in the driver circuit can also be formedby utilizing the process for forming the second transistor. Theback-gate electrode of the first transistor can be formed by utilizingthe process for forming the pixel electrode electrically connected tothe second transistor. In other words, the back-gate electrode is aconductive film which is formed using the same material as the pixelelectrode.

In the above structure, when the insulating film provided over the oxidesemiconductor film of the second transistor has a stacked-layerstructure of an oxide insulating film and a nitride insulating film, thedielectric film can have a stacked-layer structure of the oxideinsulating film and the nitride insulating film.

Moreover, when the insulating film provided over the semiconductor filmof the second transistor has a stacked-layer structure of an oxideinsulating film and a nitride insulating film, the dielectric film ofthe capacitor can have a single-layer structure of only the nitrideinsulating film by removing a region of the oxide insulating film, whichis over the capacitor. In other words, the nitride insulating film is incontact with the oxide semiconductor film serving as the one electrodeof the capacitor. A defect state (an interface state) is formed at theinterface between the nitride insulating film and the oxidesemiconductor film when the nitride insulating film and the oxidesemiconductor film are in contact with each other. Further oralternatively, when the nitride insulating film is formed by a plasmaCVD method or a sputtering method, the semiconductor film is exposed toplasma and oxygen vacancies are generated. Furthermore, nitrogen and/orhydrogen contained in the nitride insulating film are/is transferred tothe semiconductor film. Due to entry of hydrogen contained in thenitride insulating film into the defect state or an oxygen vacancy, anelectron serving as a carrier is generated. Accordingly, thesemiconductor film becomes an n-type semiconductor film with increasedconductivity; thus, a film having conductivity is obtained. Therefore,the oxide semiconductor film can sufficiently and easily serve as theone electrode of the capacitor. Moreover, charge capacity of thecapacitor can be increased because it is possible to reduce thethickness of the dielectric film.

According to another embodiment of the present invention, asemiconductor device includes a driver circuit which includes a firsttransistor including gate electrodes above and below a semiconductorfilm so as to overlap with the semiconductor film; a pixel whichincludes a second transistor including a semiconductor film; a capacitorwhich includes a dielectric film between a pair of electrodes and apixel electrode electrically connected to the second transistor, whichare in the pixel; and a capacitor line electrically connected to one ofthe pair of electrodes. In the semiconductor device, the gate electrodeover the semiconductor film of the first transistor is electricallyconnected to the capacitor line, an insulating film which has astacked-layer structure of an oxide insulating film and a nitrideinsulating film is at least over the semiconductor film of the secondtransistor, the capacitor includes a semiconductor film on the samesurface as the semiconductor film of the second transistor and thesemiconductor film serves as the one of the pair of electrodes, thepixel electrode serves as the other of the pair of electrodes, and thedielectric film is the nitride insulating film.

In the case where the insulating film provided over the oxidesemiconductor film of the second transistor has a stacked-layerstructure of an oxide insulating film and a nitride insulating film, itis preferable that the oxide insulating film be unlikely to transmitnitrogen, that is, the oxide insulating film have a barrier propertyagainst nitrogen.

With such a structure, diffusion of nitrogen into the oxidesemiconductor film of the second transistor can be suppressed, andchange in the electrical characteristics of the second transistor can besuppressed. Note that in the case where an oxide semiconductor film isused also for the first transistor, it is preferable that the oxideinsulating film be unlikely to transmit nitrogen, that is, the oxideinsulating film have a barrier property against nitrogen. Accordingly,the change in the electrical characteristics of the first transistor canbe suppressed.

In addition, the one electrode of the capacitor can be electricallyconnected to the capacitor line through a conductive film formed in theprocess for forming a source electrode and a drain electrode of thesecond transistor. Alternatively, the one electrode and the capacitorline can be electrically connected to each other by forming the oxidesemiconductor film serving as the one electrode so as to be in directcontact with the capacitor line.

Note that the conductive film which connects the one electrode of thecapacitor and the capacitor line to each other may be provided incontact with the end portion of the oxide semiconductor film serving asthe one electrode. For example, the conductive film can be provided incontact with the oxide semiconductor film along the outer peripherythereof. With such a structure, the conductivity of the oxidesemiconductor film can be increased. The oxide semiconductor film caneasily serve as the one electrode of the capacitor by increasing theconductivity of the oxide semiconductor film.

In the above structure, the capacitor line may extend in a directionparallel to a scan line serving as a gate electrode of the secondtransistor, and the capacitor line and the scan line may be provided onthe same surface. Alternatively, the capacitor line may extend in adirection parallel to a signal line serving as the source electrode orthe drain electrode of the second transistor, and the capacitor line andthe signal line may be provided on the same surface.

In the above structure, an organic insulating film may be providedbetween the pixel electrode electrically connected to the secondtransistor and the insulating film provided over the oxide semiconductorfilm of the second transistor. With such a structure, parasiticcapacitance between the pixel electrode and another conductive film, forexample, the conductive film for forming the source electrode or thedrain electrode of the second transistor can be reduced; accordingly,the electrical characteristics of the semiconductor device can be madefavorable. For example, signal delays of the semiconductor device can bereduced.

To increase the charge capacity of the capacitor in this case, it iseffective to reduce the thickness of the dielectric film; therefore, itis preferable to remove a region of the organic insulating film which isover a region where the capacitor is formed.

Further, to suppress diffusion of hydrogen, water, and the likecontained in the organic insulating film into the oxide semiconductorfilm of the second transistor, it is preferable to remove a region ofthe organic insulating film, which overlaps with the semiconductor filmof the second transistor.

In the above structure, in the case where the oxide semiconductor filmwhich is formed in the process for forming the oxide semiconductor filmof the second transistor is used as the one electrode of the capacitor,it is preferable to increase the conductivity of the oxide semiconductorfilm. That is, the one electrode of the capacitor is preferably theoxide semiconductor film which is formed on the same surface as theoxide semiconductor film of the second transistor and includes a regionhaving a higher conductivity than the oxide semiconductor film of thesecond transistor. With such a structure, the oxide semiconductor filmcan sufficiently and easily serve as the one electrode of the capacitor.

In order to increase the conductivity, it is preferable to add one ormore selected from boron, nitrogen, fluorine, aluminum, phosphorus,arsenic, indium, tin, antimony, and a rare gas element to the oxidesemiconductor film. An ion implantation method, an ion doping method, orthe like may be employed to add the element to the oxide semiconductorfilm. Alternatively, the oxide semiconductor film may be exposed toplasma containing the element so that the element can be added. In thatcase, the conductivity of the oxide semiconductor film serving as theone electrode of the capacitor is higher than or equal to 10 S/cm andlower than or equal to 1000 S/cm, preferably higher than or equal to 100S/cm and lower than or equal to 1000 S/cm.

Note that as described above, with the structure in which the nitrideinsulating film included in the insulating film which is provided overthe oxide semiconductor film serving as the one electrode of thecapacitor is in contact with the oxide semiconductor film, a step ofadding an element which increases the conductivity to the semiconductorfilm by an ion implantation method, an ion doping method, or the likecan be skipped; therefore, the yield of the semiconductor device can beincreased and the manufacturing cost thereof can be reduced.

Note that a fabrication method of a semiconductor device of oneembodiment of the present invention is also one embodiment of thepresent invention.

According to one embodiment of the present invention, in a semiconductordevice including a driver circuit, a semiconductor device including acapacitor whose charge capacity is increased while the aperture ratio isimproved can be provided. Further, in the semiconductor device includingthe driver circuit, a semiconductor device which has a high apertureratio and low power consumption and which includes a capacitor withlarge charge capacity can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram showing a semiconductor device of one embodiment ofthe present invention, and FIG. 1B is a circuit diagram of a pixelthereof.

FIGS. 2A and 2B are a top view and a cross-sectional view illustrating asemiconductor device of one embodiment of the present invention.

FIG. 3 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 4 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIGS. 5A and 5B are cross-sectional views illustrating a fabricationmethod of a semiconductor device of one embodiment of the presentinvention.

FIGS. 6A and 6B are cross-sectional views illustrating a fabricationmethod of a semiconductor device of one embodiment of the presentinvention.

FIG. 7 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 8 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIG. 9 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 10 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIG. 11 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIGS. 12A and 12B are cross-sectional views illustrating a semiconductordevice of one embodiment of the present invention.

FIG. 13 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 14 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIG. 15 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 16 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIG. 17 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIG. 18 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 19 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 20 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 21 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 22 is a cross-sectional view illustrating a transistor that can beused for a semiconductor device of one embodiment of the presentinvention.

FIG. 23 is a cross-sectional view illustrating a transistor that can beused for a semiconductor device of one embodiment of the presentinvention.

FIG. 24 is a cross-sectional view illustrating a transistor that can beused for a semiconductor device of one embodiment of the presentinvention.

FIG. 25 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 26 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIGS. 27A and 27B are cross-sectional views illustrating a fabricationmethod of a semiconductor device of one embodiment of the presentinvention.

FIGS. 28A and 28B are cross-sectional views illustrating a fabricationmethod of a semiconductor device of one embodiment of the presentinvention.

FIG. 29 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIG. 30 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

FIG. 31 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIGS. 32A and 32B are cross-sectional views illustrating a fabricationmethod of a semiconductor device of one embodiment of the presentinvention.

FIGS. 33A and 33B are cross-sectional views illustrating a fabricationmethod of a semiconductor device of one embodiment of the presentinvention.

FIG. 34 is a cross-sectional view illustrating a transistor that can beused for a semiconductor device of one embodiment of the presentinvention.

FIGS. 35A to 35C are top views each illustrating a semiconductor deviceof one embodiment of the present invention.

FIGS. 36A and 36B are cross-sectional views each illustrating asemiconductor device of one embodiment of the present invention.

FIGS. 37A to 37C are a top view and cross-sectional views illustrating asemiconductor device of one embodiment of the present invention.

FIGS. 38A to 38C are diagrams each illustrating an electronic deviceincluding a semiconductor device of one embodiment of the presentinvention.

FIGS. 39A to 39C are diagrams illustrating an electronic deviceincluding a semiconductor device of one embodiment of the presentinvention.

FIGS. 40A to 40D are diagrams illustrating structures of samples.

FIG. 41 is a graph showing sheet resistance.

FIGS. 42A and 42B show results of SIMS measurement.

FIGS. 43A to 43C are graphs showing results of ESR measurement.

FIG. 44 is a graph showing results of ESR measurement.

FIG. 45 is a graph showing sheet resistance.

FIG. 46 is a graph showing sheet resistance.

FIGS. 47A to 47D are diagrams describing bulk models of InGaZnO₄.

FIGS. 48A and 48B are a graph and a diagram describing formation energyand a thermodynamic transition level of VoH.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and examples of the present invention will be describedbelow in detail with reference to the accompanying drawings. However,the present invention is not limited to the following description and itis easily understood by those skilled in the art that the mode anddetails can be variously changed. In addition, the present inventionshould not be construed as being limited to the description in thefollowing embodiments and examples.

Note that in structures of the present invention described below, thesame portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and description thereofis not repeated. Further, the same hatching pattern is applied toportions having similar functions, and in some cases the portions arenot especially denoted by reference numerals.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is in some casesexaggerated for simplicity. Therefore, the embodiments and the examplesof the present invention are not limited to such scales.

Note that the ordinal numbers such as “first” and “second in thisspecification and the like are used for convenience and do not indicatethe order of steps or the stacking order of layers. In addition, theordinal numbers in this specification and the like do not indicateparticular names which specify the present invention.

Functions of a “source” and a “drain” in the present invention aresometimes replaced with each other when the direction of current flow ischanged in circuit operation, for example. Therefore, the terms “source”and “drain” can be used to denote the drain and the source,respectively, in this specification.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. In general, a difference between a potential of one point and areference potential (e.g., a ground potential) is merely called apotential or a voltage, and a potential and a voltage are used in manycases as synonymous words. Thus, in this specification, a potential maybe rephrased as a voltage and a voltage may be rephrased as a potentialunless otherwise specified.

In this specification, in the case where etching treatment is performedafter a photolithography process is performed, a mask formed in thephotolithography process is removed after the etching treatment.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention will be described with reference to drawings. Notethat in this embodiment, a liquid crystal display device is described asan example of the semiconductor device of one embodiment of the presentinvention.

<Structure of Semiconductor Device>

FIG. 1A shows an example of a semiconductor device. A semiconductordevice illustrated in FIG. 1A includes a pixel portion 100; a scan linedriver circuit 104; a signal line driver circuit 106; m scan lines 107which are arranged in parallel or almost in parallel to each other andwhose potentials are controlled by the scan line driver circuit 104; andn signal lines 109 which are arranged in parallel or almost in parallelto each other and whose potentials are controlled by the signal linedriver circuit 106. In addition, the pixel portion 100 includes aplurality of pixels 101 arranged in a matrix. Further, capacitor lines115 which are arranged in parallel or almost in parallel to the scanlines 107 are also included. Note that the capacitor lines may bearranged in parallel or almost in parallel to the signal lines 109. Notealso that m and n are individually an integer of 1 or more.

Each of the scan lines 107 is electrically connected to the n pixels 101arranged in the corresponding row among the plurality of pixels 101arranged in m rows and n columns in the pixel portion 100. Each signalline 109 is electrically connected to the m pixels 101 in thecorresponding column among the pixels 101 arranged in m rows and ncolumns. Each capacitor line 115 is electrically connected to the npixels 101 in the corresponding row among the pixels 101 arranged in mrows and n columns. Note that in the case where the capacitor lines 115are arranged in parallel or substantially in parallel along the signallines 109, each capacitor line 115 is electrically connected to the mpixels 101 in the corresponding column among the pixels 101 arranged inm rows and n columns.

FIG. 1B is an example of a circuit diagram of a pixel 101 included inthe semiconductor device illustrated in FIG. 1A, The pixel 101illustrated in FIG. 1B includes a transistor 103 electrically connectedto the scan line 107 and the signal line 109, a capacitor 105 oneelectrode of which is electrically connected to a capacitor line 115which supplies a constant potential and the other electrode of which iselectrically connected to a drain electrode of the transistor 103, and aliquid crystal element 108. A pixel electrode of the liquid crystalelement 108 is electrically connected to the drain electrode of thetransistor 103 and the other electrode of the capacitor 105, and anelectrode (counter electrode) facing the pixel electrode is electricallyconnected to a wiring which supplies a counter potential.

In a transistor in which a semiconductor film includes a channelformation region, the off-state current of the transistor can besignificantly reduced when an oxide semiconductor film which isprocessed under appropriate conditions is used. For this reason, anoxide semiconductor film 111 is used as a semiconductor film included inthe transistor 103.

The capacitor 105 can be formed by utilizing the process for forming thetransistor 103. The one electrode of the capacitor 105 is alight-transmitting semiconductor film, specifically an oxidesemiconductor film 119. That is, the capacitor 105 can be regarded as aMOS (metal oxide semiconductor) capacitor.

The liquid crystal element 108 is an element which controls transmissionor non-transmission of light by an optical modulation action of liquidcrystal which is sandwiched between a substrate provided with thetransistor 103 and a pixel electrode 121 and a substrate provided with acounter electrode 154. The optical modulation action of liquid crystalis controlled by an electric field (including a lateral electric field,a vertical electric field, and a diagonal electric field) applied to theliquid crystal.

The scan line driver circuit 104 and the signal line driver circuit 106are each roughly classified into a logic circuit portion, and a switchportion or a buffer portion. Although a specific structure of the scanline driver circuit 104 and the signal line driver circuit 106 isomitted here, a transistor is included in each of the scan line drivercircuit 104 and the signal line driver circuit 106.

The transistor included in one or both of the scan line driver circuit104 and the signal line driver circuit 106 can be formed utilizing theprocess for forming the transistor 103 included in the pixel 101. Thatis, one or both of the scan line driver circuit 104 and the signal linedriver circuit 106 can be provided over a substrate over which thetransistor 103 and the pixel electrode 121 in the pixel 101 areprovided. In this manner, one or both of the scan line driver circuit104 and the signal line driver circuit 106 are formed over the samesubstrate, whereby the number of components of the semiconductor devicecan be reduced and the manufacturing cost can be reduced.

Further, in order to increase the scanning speed of the pixel 101, it isnecessary to increase the operation speed of the scan line drivercircuit 104, specifically increase the operation speed of the transistorincluded in the scan line driver circuit 104, increase a drain current(an on-state current) that flows at the time of conduction of thetransistor, and increase the field-effect mobility of the transistor.The transistor included in the scan line driver circuit 104 is adual-gate transistor in order to achieve the above increases. Note thatthe dual-gate transistor, in which gate electrodes are provided aboveand below a semiconductor film so as to overlap with the semiconductorfilm, can achieve the increase in operation speed. Further, an electricfield is applied from both above and below the semiconductor film;therefore, the transistor can achieve the increase in on-state currentand field-effect mobility. Note that the transistor included in thesignal line driver circuit 106 can also be a dual-gate transistor.

Next, in the semiconductor device of one embodiment of the presentinvention, a stacked-layer structure of the capacitor line provided inthe pixel portion 100 and a wiring including a back-gate electrode ofthe dual-gate transistor included in the scan line driver circuit 104 isdescribed. FIGS. 2A and 2B illustrate the stacked-layer structure. FIG.2A is a top view of part of the semiconductor device, and FIG. 2B is across-sectional view taken along dashed-dotted line X1-X2 anddashed-dotted line Y1-Y2 in FIG. 2A.

FIG. 2A illustrates the pixel portion 100, the capacitor line 115, thescan line driver circuit 104, and a wiring 92 including a back-gateelectrode of a dual-gate transistor 15 included in the scan line drivercircuit 104. Note that components other than the capacitor line 115 inthe pixel portion 100 (e.g., the pixel, the signal line, the transistor,the capacitor, and the liquid crystal element) are omitted asappropriate for simplicity of the drawing. Moreover, the components ofthe scan line driver circuit 104 are omitted as appropriate forsimplicity of the drawing.

As illustrated in FIG. 2A, in the semiconductor device of one embodimentof the present invention, the capacitor line 115 and the wiring 92 areelectrically connected to each other through an opening 94. That is, thecapacitor line 115 and the wiring 92 have the same potential, andfurther the back-gate electrode of the dual-gate transistor 15 and theoxide semiconductor film 119 serving as the one electrode of thecapacitor 105 which is electrically connected to the capacitor line 115have the same potential (see FIG. 1B).

FIG. 2B illustrates the structures of the dual-gate transistor 15 andthe opening 94. Note that the dual-gate transistor 15 can be formed byutilizing the process for forming the transistor 103.

As illustrated in FIG. 2B, in the cross section taken along X1-X2 of thesemiconductor device of one embodiment of the present invention, a gateelectrode 17 a is over a substrate 102; a gate insulating film 12 isover the gate electrode 17 a; an oxide semiconductor film 11 is over aregion of the gate insulating film 12, which overlaps with the gateelectrode 17 a; a source electrode 19 a and a drain electrode 13 a arein contact with the oxide semiconductor film 11; an insulating film 29,an insulating film 31, and an insulating film 32 are over the gateinsulating film 12, the source electrode 19 a, the oxide semiconductorfilm 11, and the drain electrode 13 a; and the wiring 92 including theback-gate electrode is over a region of the insulating film 32, whichoverlaps with the oxide semiconductor film 11.

Moreover, in the cross section taken along Y1-Y2 of the semiconductordevice of one embodiment of the present invention, the capacitor line115 which is on the same surface as the gate electrode 17 a is over thesubstrate 102; a conductive film 16 and the insulating films 29, 31, and32 are over the capacitor line 115; the opening 94 reaching theconductive film 16 is in the gate insulating film 12 and the insulatingfilms 29, 31, and 32; and the wiring 92 is in the opening 94. Note thatthe capacitor line 115 and the wiring 92 are electrically connected toeach other through the conductive film 16, and the conductive film 16can be formed by utilizing the process for forming the source electrode19 a and the drain electrode 13 a.

Note that as the structure in which the capacitor line 115 and thewiring 92 are electrically connected to each other, a structure in whichthe capacitor line 115 and the wiring 92 are in direct contact with eachother without the conductive film 16 provided therebetween can beemployed as well as the above structure in which electrical connectionis obtained through the conductive film 16.

Note that a base insulating film may be provided between the substrate102, and the gate electrode 17 a, the capacitor line 115, and the gateinsulating film 12.

Although the structure in which the capacitor line 115 and the wiring 92including the back-gate electrode of the dual-gate transistor 15 whichis included in the scan line driver circuit 104 are electricallyconnected to each other is described here, the semiconductor device ofone embodiment of the present invention is not limited to thisstructure, and the capacitor line 115 can be electrically connected to awiring including a back-gate electrode of a dual-gate transistor whichis included in the signal line driver circuit 106.

Accordingly, the capacitor line 115 and the wiring 92 including theback-gate electrode of the dual-gate transistor 15 are electricallyconnected to each other in the semiconductor device of one embodiment ofthe present invention, thus the potential of the back-gate electrode canbe controlled by controlling the potential of the capacitor line 115.That is, according to one embodiment of the present invention, astructure which is configured to control the potential of the back-gateelectrode can be omitted, and the transistor included in the scan linedriver circuit 104 can be operated as the dual-gate transistor 15;accordingly, the operation speed of the driver circuit can be increased.Accordingly, a semiconductor device which can achieve both an increaseof the operation speed and a reduction of power consumption can beobtained. Further, since the structure which is configured to controlthe potential of the back-gate electrode can be omitted, the number ofcomponents of the semiconductor device can be reduced and themanufacturing cost of the semiconductor device of one embodiment of thepresent invention can be reduced.

Next, a specific structure example of the pixel 101 included in thesemiconductor device of one embodiment of the present invention isdescribed. FIG. 3 is a top view of the pixel 101. Note that in FIG. 3,some components of the semiconductor device (e.g., the liquid crystalelement 108) are omitted for simplicity of the drawing.

In FIG. 3, the scan line 107 extends in a direction substantiallyperpendicular to the signal line 109 (in the horizontal direction in thedrawing). The signal line 109 extends in a direction substantiallyperpendicular to the scan line 107 (in the vertical direction in thedrawing). The capacitor line 115 extends in a direction parallel to thescan line 107. The scan line 107 and the capacitor line 115 areelectrically connected to the wiring 92 including the back-gateelectrode of the dual-gate transistor 15 included in the scan linedriver circuit 104 (see FIG. 2A), and the signal line 109 iselectrically connected to the signal line driver circuit 106 (see FIG.1A).

The transistor 103 is provided in a region where the scan line 107 andthe signal line 109 intersect with each other. The transistor 103includes at least the oxide semiconductor film 111 including a channelformation region, a gate electrode, a gate insulating film (notillustrated in FIG. 3), a source electrode, and a drain electrode.

Since the transistor 103 includes the oxide semiconductor film 111, theoff-state current of the transistor can be significantly reduced, andthe power consumption of the semiconductor device can be reduced.

In addition, the scan line 107 includes a region serving as the gateelectrode of the transistor 103, and the signal line 109 includes aregion serving as the source electrode of the transistor 103. Aconductive film 113 includes a region serving as the drain electrode ofthe transistor 103 and is electrically connected to the pixel electrode121 through an opening 117. In FIG. 3, the hatch pattern of the pixelelectrode 121 is not illustrated.

The region of the scan line 107, which serves as the gate electrode, isa region overlapping with at least the oxide semiconductor film 111. Theregion of the signal line 109, which serves as the source electrode, isa region overlapping with at least the oxide semiconductor film 111. Theregion of the conductive film 113, which serves as the drain electrode,is a region overlapping with at least the oxide semiconductor film 111.Note that in the description below, in some cases, the gate electrode ofthe transistor 103 is described as a gate electrode 107 a, the sourceelectrode of the transistor 103 is described as a source electrode 109a, and the drain electrode of the transistor 103 is described as a drainelectrode 113 a. Further, in some cases, the term “scan line 107” isused also to denote the gate electrode of the transistor 103, and theterm “signal line 109” is used also to denote the source electrode ofthe transistor 103.

The capacitor 105 is provided in a region of the pixel 101, which issurrounded by the capacitor lines 115 and the signal lines 109. Thecapacitor 105 is electrically connected to the capacitor line 115through a conductive film 125 provided in and over an opening 123. Thecapacitor 105 includes the light-transmitting oxide semiconductor film119, the light-transmitting pixel electrode 121, and, as a dielectricfilm, light-transmitting insulating films (not illustrated in FIG. 3)which are included in the transistor 103. That is, the capacitor 105transmits light.

Owing to the light-transmitting property of the capacitor 105, thecapacitor 105 can be formed large (in a large area) in the pixel 101.For this reason, the semiconductor device can have charge capacityincreased while the aperture ratio is improved. Thus, the semiconductordevice can have an excellent display quality.

Charge capacity accumulated in the capacitor 105 is changed depending onthe overlapped area of a pair of electrodes. When the size of a pixel isreduced in order to increase the resolution, the size of a capacitor isalso reduced, resulting in a small accumulated charge capacity.Accordingly, a liquid crystal element might not be operatedsufficiently. Since the capacitor 105 transmits light, the capacitor 105can be formed in the entire operation area of the liquid crystal element108, and thus the capacitor 105 can be formed large (in a large area) asmuch as possible in the pixel. As long as the charge capacity that cansufficiently operate the liquid crystal element 108 can be ensured, thepixel density can be increased to have a high resolution.

Thus, according to one embodiment of the present invention, thecapacitor 105 can be favorably used in a high-resolution display devicewith a pixel density of 200 ppi or more, further 300 ppi or more.Further, according to one embodiment of the present invention, theaperture ratio can be improved even in a display device with a highresolution, which makes it possible to use efficiently light from alight source device such as a backlight, so that power consumption ofthe display device can be reduced.

Here, the characteristics of a transistor including an oxidesemiconductor is described. Note that the transistor including an oxidesemiconductor is an n-channel transistor. Oxygen vacancies in an oxidesemiconductor might generate carriers, which might lower the electricalcharacteristics and the reliability of the transistor. For example, insome cases, the threshold voltage of the transistor is shifted in thenegative direction, and drain current flows when the gate voltage is 0V. The characteristics of a transistor in which drain current flows whenthe gate voltage is 0 V is referred to as a normally-on characteristics,whereas the characteristics of a transistor in which substantially nodrain current flows when the gate voltage is 0 V is referred to as anormally-off characteristics.

In view of the above, it is preferable that defects, typically oxygenvacancies in an oxide semiconductor film be reduced as much as possiblewhen the oxide semiconductor film is used. For example, the spin densityof the oxide semiconductor film (the density of defects in the oxidesemiconductor film) at a g-value of 1.93 in electron spin resonance inwhich a magnetic field is applied in parallel to the film surface ispreferably reduced to be lower than or equal to the lower limit ofdetection of a measurement instrument. When the defects, typically theoxygen vacancies in the oxide semiconductor film are reduced as much aspossible, it is possible to suppress having normally-on characteristicsof the transistor, leading to improvement in the electricalcharacteristics and reliability of a semiconductor device.

The shift of the threshold voltage of a transistor in the negativedirection is caused in some cases by hydrogen (including a hydrogencompound such as water) contained in an oxide semiconductor film as wellas by oxygen vacancies. Hydrogen contained in the oxide semiconductorfilm is reacted with oxygen bonded to a metal atom to be water, and inaddition, vacancies (also referred to as oxygen vacancies) are formed ina lattice from which oxygen is released (or a portion from which oxygenis released). In addition, part of hydrogen reacts with oxygen, whichcauses generation of electrons serving as carriers. Thus, a transistorincluding an oxide semiconductor film which contains hydrogen is likelyto have normally-on characteristics.

Accordingly, it is preferable that hydrogen be reduced as much aspossible in the oxide semiconductor film 111 of the transistor 103.Specifically, the hydrogen concentration of the oxide semiconductor film111, which is measured by secondary ion mass spectrometry (SIMS), islower than 5×10¹⁸ atoms/cm³, preferably lower than or equal to 1×10¹⁸atoms/cm³, more preferably lower than or equal to 5×10¹⁷ atoms/cm³,further preferably lower than or equal to 1×10¹⁶ atoms/cm³.

Further, the concentration of alkali metals or alkaline earth metals inthe oxide semiconductor film 111, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³. When an alkali metal and an alkaline earth metal arebonded to an oxide semiconductor, a carrier might be generated, whichmight cause an increase in the off-state current of the transistor 103.

Further, when nitrogen is contained in the oxide semiconductor film,electrons serving as carriers are generated and the carrier densityincreases, so that the oxide semiconductor film easily becomes n-type.As a result, a transistor including the oxide semiconductor film whichcontains nitrogen is likely to have normally-on characteristics. Forthis reason, nitrogen in the oxide semiconductor film 111 is preferablyreduced as much as possible; the nitrogen concentration is preferablylower than or equal to 5×10¹⁸ atoms/cm³, for example.

Further, when a Group 14 element such as silicon and carbon is containedin the oxide semiconductor film, electrons serving as carriers aregenerated and the carrier density increases, so that the oxidesemiconductor film easily becomes n-type. Thus, in the transistor 103including the oxide semiconductor film 111, in particular, at theinterface between a gate insulating film 127 (not illustrated in FIG. 3)and the oxide semiconductor film 111, the silicon concentration which ismeasured by SIMS is lower than or equal to 3×10¹⁸ atoms/cm³, preferablylower than or equal to 3×10¹⁷ atoms/cm³. Note that at the interface, thecarbon concentration measured by SIMS is lower than or equal to 3×10¹⁸atoms/cm³, preferably lower than or equal to 3×10¹⁷ atoms/cm³.

As described above, when the oxide semiconductor film 111 which ishighly purified by reducing impurities (such as hydrogen, nitrogen,silicon, carbon, an alkali metal, and an alkaline earth metal) as muchas possible is used, it is possible to suppress having normally-oncharacteristics of the transistor 103, so that the off-state current ofthe transistor 103 can be significantly reduced. Therefore, asemiconductor device having favorable electrical characteristics can befabricated. Further, a highly reliable semiconductor device can befabricated.

Note that various experiments can prove low off-state current of atransistor including a highly purified oxide semiconductor film. Forexample, even when an element has a channel width W of 1×10⁶ μm and achannel length L of 10 μm, the off-state current can be lower than orequal to the measurement limit of a semiconductor parameter analyzer,i.e., lower than or equal to 1×10⁻¹³ A, at voltages (drain voltages)between a source and a drain of 1 V to 10 V. In that case, it is foundthat the off-state current corresponding to a value obtained by dividingthe off-state current by the channel width of the transistor is lowerthan or equal to 100 zA/μm. Further, the off-state current was measuredwith the use of a circuit in which a capacitor and a transistor areconnected to each other and charge that flows in or out from thecapacitor is controlled by the transistor. In the measurement, apurified oxide semiconductor film has been used for a channel formationregion of the transistor, and the off-state current of the transistorhas been measured from change in the amount of charge of the capacitorper unit time. As a result, it is found that in the case where thevoltage between the source electrode and the drain electrode of thetransistor is 3 V, lower off-state current of several tens ofyoctoamperes per micrometer (yA/μm) can be obtained. Accordingly, thetransistor including a highly purified oxide semiconductor film hasextremely low off-state current.

Next, FIG. 4 is a cross-sectional view taken along dashed-dotted lineA1-A2 and dashed-dotted line B1-B2 in FIG. 3. Note that FIG. 4illustrates a structure including the liquid crystal element 108.

The cross-sectional structure of the pixel 101 is as follows. Over thesubstrate 102, the scan line 107 including the gate electrode 107 a andthe capacitor line 115 which is on the same surface as the scan line 107are provided. The gate insulating film 127 is provided over the scanline 107 and the capacitor line 115. The oxide semiconductor film 111 isprovided over a region of the gate insulating film 127, which overlapswith the scan line 107, and the oxide semiconductor film 119 is providedover another region of the gate insulating film 127. The signal line 109including the source electrode 109 a and the conductive film 113 servingas the drain electrode 113 a are provided over the oxide semiconductorfilm 111 and the gate insulating film 127. In the gate insulating film127, the opening 123 reaching the capacitor line 115 is provided, andthe conductive film 125 is provided in and over the opening 123 and incontact with the capacitor line 115 and the oxide semiconductor film119. An insulating film 129, an insulating film 131, and an insulatingfilm 132 which each serve as a protective insulating film of thetransistor 103 are provided over the gate insulating film 127, thesignal line 109, the oxide semiconductor film 111, the conductive film113, the conductive film 125, and the oxide semiconductor film 119. Theopening 117 reaching the conductive film 113 is provided in theinsulating films 129, 131, and 132, and the pixel electrode 121 isprovided in and over the opening 117. Note that a base insulating filmmay be provided between the substrate 102, and the scan line 107, thecapacitor line 115, and the gate insulating film 127.

Further, the pixel 101 includes the liquid crystal element 108. Thecross-sectional structure of the liquid crystal element 108 is asfollows. On a surface of a substrate 150, which faces the substrate 102,a light-blocking film 152 is provided in a region overlapping with atleast the transistor 103, the counter electrode 154 which is alight-transmitting conductive film is provided so as to cover thelight-blocking film 152, and an alignment film 156 is provided so as tocover the light-blocking film 152 and the counter electrode 154. Analignment film 158 is provided over the pixel electrode 121 and theinsulating film 132. Liquid crystal 160 is provided between thesubstrate 102 and the substrate 150. The liquid crystal 160 is incontact with the alignment film 156 provided on the substrate 150 sideand the alignment film 158 provided on the substrate 102 side.

Note that in the case where the semiconductor device of one embodimentof the present invention is a liquid crystal display device, a lightsource device such as a backlight; an optical member (an opticalsubstrate) such as a polarizing plate, which is provided on thesubstrate 102 side and the substrate 150 side; a sealant for fixing thesubstrate 102 and the substrate 150; and the like are needed. Thesecomponents will be described later.

In the capacitor 105 described in this embodiment, the oxidesemiconductor film 119 serves as one of a pair of electrodes, the pixelelectrode 121 serves as the other of the pair of electrodes, and theinsulating films 129, 131, and 132 serve as a dielectric film providedbetween the pair of electrodes.

Here, an operation principle of the capacitor 105 is described.

Despite having a structure which is the same as that of the oxidesemiconductor film 111, the oxide semiconductor film 119 serves as oneelectrode of the capacitor 105. This is because the pixel electrode 121can serve as a gate electrode, the insulating films 129, 131, and 132can serve as gate insulating films, and the capacitor line 115 can serveas a source electrode or a drain electrode, so that the capacitor 105can be operated in a manner similar to that of a transistor and theoxide semiconductor film 119 can be brought into conduction. That is,the capacitor 105 can be a MOS capacitor, and the oxide semiconductorfilm 119 can be brought into conduction so that the oxide semiconductorfilm 119 can serve as one electrode of the capacitor by controlling apotential applied to the capacitor line 115. In that case, the potentialapplied to the capacitor line 115 is set as follows. The potential ofthe pixel electrode 121 is shifted in the positive direction and thenegative direction in order to operate the liquid crystal element 108.The potential of the capacitor line 115 needs to be constantly lowerthan the potential applied to the pixel electrode 121 by the thresholdvoltage of the capacitor 105 (MOS capacitor) or more in order that thecapacitor 105 (MOS capacitor) be constantly in a conductive state. Thatis, since the oxide semiconductor film 119 has the same structure as theoxide semiconductor film 111, the potential of the capacitor line 115should be lower than the potential applied to the pixel electrode 121 bythe threshold voltage of the transistor 103 or more. A channel is formedin the oxide semiconductor film 119 in such a manner; therefore, acapacitor 305 (MOS capacitor) can be constantly brought into conduction.

The details of the components of the above structure will be describedbelow.

There is no particular limitation on the property of a material and thelike of the substrate 102 as long as the material has heat resistanceenough to withstand at least heat treatment performed in a fabricationprocess of the semiconductor device. Examples of the substrate are aglass substrate, a ceramic substrate, and a plastic substrate, and asthe glass substrate, an alkali-free glass substrate such as a bariumborosilicate glass substrate, an aluminoborosilicate glass substrate, oran aluminosilicate glass substrate is preferably used. Alternatively, anon-light-transmitting substrate such as a stainless alloy substrate maybe used, in which case a surface of the substrate is preferably providedwith an insulating film. As the substrate 102, any of the following mayalternatively be used: a quartz substrate, a sapphire substrate, asingle crystal semiconductor substrate, a polycrystalline semiconductorsubstrate, a compound semiconductor substrate, and a silicon oninsulator (SOI) substrate.

The scan line 107 and the capacitor line 115, through which a largeamount of current flows, are preferably formed to have a single-layerstructure or a stacked-layer structure using a metal film, typically anyof metal materials such as molybdenum (Mo), titanium (Ti), tungsten (W),tantalum (Ta), aluminum (Al), copper (Cu), chromium (Cr), neodymium(Nd), or scandium (Sc), or an alloy material which contains any of thesematerials as its main component.

Examples of the scan line 107 and the capacitor line 115 are asingle-layer structure using aluminum containing silicon, a two-layerstructure in which titanium is stacked over aluminum, a two-layerstructure in which titanium is stacked over a titanium nitride, atwo-layer structure in which tungsten is stacked over a titaniumnitride, a two-layer structure in which tungsten is stacked over atantalum nitride, a two-layer structure in which copper is stacked overa copper-magnesium-aluminum alloy, and a three-layer structure in whichtitanium nitride, copper, and tungsten are stacked in this order.

As a material of the scan line 107 and the capacitor line 115, alight-transmitting conductive material that can be used for the pixelelectrode 121 can be used. Note that in the case where the semiconductordevice of one embodiment of the present invention is a reflectivedisplay device, a non-light-transmitting conductive material (e.g., ametal material) can be used for the pixel electrode 121. In that case,similarly, a non-light-transmitting substrate can be used as thesubstrate 102.

Further, as the material of the scan line 107 and the capacitor line115, a metal oxide containing nitrogen, specifically, an In—Ga—Zn-basedoxide containing nitrogen, an In—Sn-based oxide containing nitrogen, anIn—Ga-based oxide containing nitrogen, an In—Zn-based oxide containingnitrogen, a Sn-based oxide containing nitrogen, an In-based oxidecontaining nitrogen, or a metal nitride (InN, SnN, or the like) film canbe used. These materials each have a work function higher than or equalto 5 eV (electron volts). The use of a metal oxide containing nitrogenfor the scan line 107 (the gate electrode 107 a) allows the thresholdvoltage of the transistor 103 to be shifted in the positive direction,i.e. the transistor can have normally-off characteristics. For example,in the case where an In—Ga—Zn-based oxide containing nitrogen is used,an In—Ga—Zn-based oxide having at least a higher nitrogen concentrationthan the oxide semiconductor film 111, specifically an In—Ga—Zn-basedoxide having a nitrogen concentration of 7 at. % or higher can be used.

It is preferable to use aluminum or copper which is a low-resistantmaterial for the scan line 107 and the capacitor line 115. When aluminumor copper is used, signal delay is reduced, so that the display qualitycan be improved. Note that aluminum has low heat resistance; therefore,defects due to a hillock, a whisker, or migration tend to be caused. Inorder to prevent migration of aluminum, a stacked-layer structureincluding aluminum and a metal material having a higher melting pointthan aluminum, such as molybdenum, titanium, or tungsten, is preferablyused. Also when copper is used, in order to prevent a defect due tomigration and diffusion of copper elements, a stacked-layer structureincluding copper and a metal material having a higher melting point thancopper, such as molybdenum, titanium, or tungsten, is preferably used.

Further, as illustrated in FIG. 3 and FIG. 4, it is preferable that thescan line 107 (the gate electrode 107 a) be provided to have a shapesuch that the oxide semiconductor film 111 can be provided in the regionof the scan line 107 (the gate electrode 107 a). As illustrated in FIG.3, the oxide semiconductor film 111 is preferably provided on the innerside of the scan line 107. In this manner, light which enters from asurface of the substrate 102, which is opposite to a surface on whichthe scan line 107 is provided, (the rear surface of the substrate 102)(in a liquid crystal display device, such light corresponds to lightfrom a light source device such as a backlight) is shielded by the scanline 107; therefore, change or degradation of the electricalcharacteristics of the transistor 103 (e.g., threshold voltage) can besuppressed.

The gate insulating film 127 is provided to have a single-layerstructure or a stacked-layer structure using, for example, one or moreof insulating materials such as silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, aluminum oxide, hafnium oxide,gallium oxide, or Ga—Zn-based metal oxide. Note that in order to improvethe characteristics of the interface between the gate insulating film127 and the oxide semiconductor film 111, a region in the gateinsulating film 127, which is in contact with at least the oxidesemiconductor film 111, is preferably formed using an oxide insulatingfilm.

It is possible to prevent outward diffusion of oxygen contained in theoxide semiconductor film 111 and entry of hydrogen, water, or the likeinto the oxide semiconductor film 111 from the outside by providing aninsulating film having a barrier property against oxygen, hydrogen,water, and the like for the gate insulating film 127. As for theinsulating film having a barrier property against oxygen, hydrogen,water, and the like, an aluminum oxide film, an aluminum oxynitridefilm, a gallium oxide film, a gallium oxynitride film, an yttrium oxidefilm, an yttrium oxynitride film, a hafnium oxide film, a hafniumoxynitride film, and a silicon nitride film can be given as examples.

The gate insulating film 127 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate containing nitrogen(HfSi_(x)O_(y)N_(z)), hafnium aluminate containing nitrogen(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor 103 can be reduced.

Moreover, it is preferable that the gate insulating film 127 have astacked-layer structure including the following: a silicon nitride filmwith a small number of defects as a first silicon nitride film; asilicon nitride film with small amounts of released hydrogen andreleased ammonia as a second silicon nitride film over the first siliconnitride film; and any one of the oxide insulating films given above asexamples of the gate insulating film 127 over the second silicon nitridefilm.

In the second silicon nitride film, in thermal desorption spectrometry,the number of released hydrogen molecules is preferably less than 5×10²¹molecules/cm³, more preferably less than or equal to 3×10²¹molecules/cm³, further preferably less than or equal to 1×10²¹molecules/cm³, and the number of released ammonia molecules ispreferably less than 1×10²² molecules/cm³, more preferably less than orequal to 5×10²¹ molecules/cm³, further preferably less than or equal to1×10²¹ molecules/cm³. The first silicon nitride film and the secondsilicon nitride film are used as part of the gate insulating film 127,so that a gate insulating film with a small number of defects and smallamounts of released hydrogen and released ammonia can be formed as thegate insulating film 127. Accordingly, it is possible to reduce theamount of hydrogen and nitrogen in the gate insulating film 127, whichare transferred to the oxide semiconductor film 111.

In the case where the trap level (also referred to as interface level)is present at the interface between an oxide semiconductor film and agate insulating film or in the gate insulating film in a transistorincluding an oxide semiconductor, change of the threshold voltage,typically change of the threshold voltage in the negative direction inthe transistor and an increase in the subthreshold swing (S value)showing a gate voltage needed for changing the drain current by onedigit when the transistor is turned on are caused. Thus, there is aproblem in that electrical characteristics fluctuate among thetransistors. For this reason, when, as the gate insulating film 127, thesilicon nitride film with a small number of defects is used, and theoxide insulating film is provided in a region of the gate insulatingfilm 127, which is in contact with the oxide semiconductor film 111, anegative shift of the threshold voltage and an increase of an S valuecan be suppressed.

The thickness of the gate insulating film 127 is greater than or equalto 5 nm and less than or equal to 400 nm, preferably greater than orequal to 10 nm and less than or equal to 300 nm, more preferably greaterthan or equal to 50 nm and less than or equal to 250 nm.

The oxide semiconductor film 111 can have an amorphous crystalstructure, a single crystal structure, or a polycrystalline structure.The thickness of the oxide semiconductor film 111 is greater than orequal to 1 nm and less than or equal to 100 nm, preferably greater thanor equal to 1 nm and less than or equal to 30 nm, more preferablygreater than or equal to 1 nm and less than or equal to 50 nm, furtherpreferably greater than or equal to 3 nm and less than or equal to 20nm.

Further, an oxide semiconductor that can be used for the oxidesemiconductor film 111 has an energy gap of greater than or equal to 2eV, preferably greater than or equal to 2.5 eV, more preferably greaterthan or equal to 3 eV. The off-state current of the transistor 103 canbe reduced by using an oxide semiconductor with a wide energy gap inthis manner.

An oxide semiconductor containing at least indium (In) or zinc (Zn) ispreferably used for the oxide semiconductor film 111. Alternatively,both In and Zn are preferably contained. In order to reduce fluctuationsin electrical characteristics of the transistors including the oxidesemiconductor, the oxide semiconductor preferably contains one or moreof stabilizers in addition to In and Zn.

As for stabilizers, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al),and zirconium (Zr) can be given as examples. As another stabilizer,lanthanoids such as lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and lutetium (Lu) can be given as examples.

As the oxide semiconductor that can be used for the oxide semiconductorfilm 111, for example, the following can be used: indium oxide, tinoxide, or zinc oxide; an oxide containing two kinds of metals such as anIn—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, aZn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or anIn—Ga-based oxide; an oxide containing three kinds of metals such as anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, anAl—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide,an In—Zr—Zn-based oxide, an In—Ti—Zn-based oxide, In—Sc—Zn-based oxide,an In—Y—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-basedoxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; or an oxide containingfour kinds of metals such as an In—Sn—Ga—Zn-based oxide, anIn—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn— metal oxide, or anIn—Hf—Al—Zn-based oxide.

Here, an In—Ga—Zn-based oxide means an oxide containing In, Ga, and Znas its main components and there is no particular limitation on theratio of In, Ga, and Zn. The In—Ga—Zn-based oxide may contain a metalelement other than In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0 issatisfied) may be used as the oxide semiconductor. Note that Mrepresents one or more metal elements selected from Ga, Fe, Mn, and Co,or the above element as a stabilizer.

For example, it is possible to use an In—Ga—Zn-based metal oxidecontaining In, Ga, and Zn at an atomic ratio of 1:1:1 (=⅓:⅓:⅓), 2:2:1(=⅖:⅖:⅕), or 3:1:2 (=½:⅙:⅓). Alternatively, an In—Sn—Zn-based metaloxide containing In, Sn, and Zn at an atomic ratio of 1:1:1 (=⅓:⅓:⅓),2:1:3 (=⅓:⅙:½), or 2:1:5 (=¼:⅛:⅝) may be used. Note that a proportion ofeach atom in the atomic ratio of the metal elements varies within arange of ±20% as an error.

However, the atomic ratio is not limited to those described above, and amaterial having the appropriate atomic ratio may be used depending onneeded semiconductor characteristics and electrical characteristics(e.g., field-effect mobility and threshold voltage). In order to obtainneeded semiconductor characteristics, it is preferable that the carrierdensity, the impurity concentration, the defect density, the atomicratio of a metal element and oxygen, the interatomic distance, thedensity, and the like be set to be appropriate. For example, a highfield-effect mobility can be obtained relatively easily in the casewhere the In—Sn—Zn-based metal oxide is used. However, the field-effectmobility can be increased by reducing the defect density in the bulkalso in the case where the In—Ga—Zn-based metal oxide is used.

For the oxide semiconductor film 119, an oxide semiconductor that can beused for the oxide semiconductor film 111 can be used. The oxidesemiconductor film 119 can be formed concurrently with the oxidesemiconductor film 111 and thus contains a metal element of an oxidesemiconductor included in the oxide semiconductor film 111.

The signal line 109, the conductive film 113, and the conductive film125 electrically connecting the oxide semiconductor film 119 and thecapacitor line 115 in the capacitor 105 can be formed to have asingle-layer structure or a stacked-layer structure using a materialthat can be used for the scan line 107 and the capacitor line 115.

The insulating films 129, 131, and 132 which each serve as theprotective insulating film of the transistor 103 and which serve as thedielectric film of the capacitor 105 are insulating films each formedusing a material that can be used for the gate insulating film 127. Itis particularly preferable that the insulating films 129 and 131 beoxide insulating films and the insulating film 132 be a nitrideinsulating film. Further, the use of a nitride insulating film as theinsulating film 132 can suppress entry of impurities such as hydrogenand water into the transistor 103 (in particular, the oxidesemiconductor film 111) from the outside. Note that the insulating film129 is not necessarily provided.

In addition, one or both of the insulating film 129 and the insulatingfilm 131 are each preferably an oxide insulating film containing oxygenat a higher proportion than oxygen in the stoichiometric composition. Inthat case, oxygen can be prevented from being released from the oxidesemiconductor film 111, and the oxygen contained in an oxygen excessregion can be transferred to the oxide semiconductor film 111 tocompensate oxygen vacancies. For example, when an oxide insulating filmhaving the following feature is used, the oxygen vacancies in the oxidesemiconductor film 111 can be compensated. The feature of the oxideinsulating film is that the number of oxygen molecules released from theoxide insulating film is greater than or equal to 1.0×10¹⁸ molecules/cm³when measured by thermal desorption spectroscopy (hereinafter referredto as TDS spectroscopy). Note that an oxide insulating film partlyincluding a region which contains oxygen at a higher proportion thanoxygen in the stoichiometric composition (oxygen excess region) may beused as one or both of the insulating film 129 and the insulating film131. When such an oxygen excess region is present in a regionoverlapping with at least the oxide semiconductor film 111, oxygen isprevented from being released from the oxide semiconductor film 111 andthe oxygen contained in the oxygen excess region can be transferred tothe oxide semiconductor film 111 to compensate oxygen vacancies.

In the case where the insulating film 131 is an oxide insulating filmcontaining oxygen at a higher proportion than oxygen in thestoichiometric composition, the insulating film 129 is preferably anoxide insulating film which transmits oxygen. Oxygen which enters theinsulating film 129 from the outside does not completely transmit theinsulating film 129 and transfer and part thereof remains in theinsulating film 129. Further, there is oxygen which is contained in theinsulating film 129 from the first and is transferred from theinsulating film 129 to the outside. Thus, the insulating film 129 ispreferably an oxide insulating film having a high coefficient ofdiffusion of oxygen.

Since the insulating film 129 is in contact with the oxide semiconductorfilm 111, the insulating film 129 is preferably an oxide insulating filmthrough which oxygen is transmitted and which has a low interface stateat the interface with the oxide semiconductor film 111. For example, theinsulating film 129 is preferably an oxide insulating film having alower defect density than the insulating film 131. Specifically, thespin density of the insulating film 129 at a g-value of 2.001(Er-center), which is measured by electron spin resonance, is lower thanor equal to 3.0×10¹⁷ spins/cm³, preferably lower than or equal to5.0×10¹⁶ spins/cm³. Note that the spin density at a g-value of 2.001,which is measured by electron spin resonance, corresponds to the numberof dangling bonds contained in the insulating film 129.

The thickness of the insulating film 129 can be greater than or equal to5 nm and less than or equal to 150 nm, preferably greater than or equalto 5 nm and less than or equal to 50 nm, more preferably greater than orequal to 10 nm and less than or equal to 30 nm. The thickness of theinsulating film 131 can be greater than or equal to 30 nm and less thanor equal to 500 nm, preferably greater than or equal to 150 nm and lessthan or equal to 400 nm.

When an oxide insulating film which transmits oxygen and which has fewerinterface states between the oxide semiconductor film 111 and the oxideinsulating film is used as the insulating film 129 provided over theoxide semiconductor film 111, and an oxide insulating film whichincludes an oxygen excess region or an oxide insulating film containingoxygen at a higher proportion than oxygen in the stoichiometriccomposition is used as the insulating film 131, oxygen can be easilysupplied to the oxide semiconductor film 111, the release of oxygen fromthe oxide semiconductor film 111 can be prevented, and the oxygencontained in the insulating film 131 can be transferred to the oxidesemiconductor film 111 to compensate the oxygen vacancies in the oxidesemiconductor film 111. Thus, it is possible to suppress havingnormally-on characteristics of the transistor 103 and a potentialapplied to the capacitor line 115 can be controlled so that thecapacitor 105 (MOS capacitor) can be constantly in a conductive state;thus, the semiconductor device can have favorable electricalcharacteristics and high reliability.

In the case where a nitrogen-containing oxide insulating film, such as asilicon oxynitride film or a silicon nitride oxide film, is used as oneor both of the insulating film 129 and the insulating film 131, thenitrogen concentration measured by SIMS is greater than or equal to thelower limit of detection by SIMS and less than 3×10²⁰ atoms/cm³,preferably greater than or equal to 1×10¹⁸ atoms/cm³ and less than orequal to 1×10²⁰ atoms/cm³. In that case, the amount of nitrogen which istransferred to the oxide semiconductor film 111 included in thetransistor 103 can be reduced and the number of defects in thenitrogen-containing oxide insulating film itself can be reduced.

In the case where a nitride insulating film is used as the insulatingfilm 132, an insulating film having a barrier property against nitrogenis preferably used as one or both of the insulating film 129 and theinsulating film 131. For example, a dense oxide insulating film can havea barrier property against nitrogen. Specifically, it is preferable touse an oxide insulating film which can be etched at a rate of less thanor equal to 10 nm per minute when the temperature is 25° C. and 0.5 wt %of fluoric acid is used.

As the insulating film 132, a nitride insulating film with a lowhydrogen content can be provided. The nitride insulating film is asfollows, for example: the number of hydrogen molecules released from thenitride insulating film is less than 5.0×10²¹ molecules/cm³, preferablyless than 3.0×10²¹ molecules/cm³, more preferably less than 1.0×10²¹molecules/cm³ when measured by TDS spectroscopy.

The insulating film 132 has a thickness with which entry of impuritiessuch as hydrogen and water from the outside can be suppressed. Forexample, the thickness can be greater than or equal to 50 nm and lessthan or equal to 200 nm, preferably greater than or equal to 50 nm andless than or equal to 150 nm, and more preferably greater than or equalto 50 nm and less than or equal to 100 nm.

The use of a nitride insulating film as the insulating film 132 providedover the insulating film 131 can suppress entry of impurities such ashydrogen and water into the oxide semiconductor film 111 and the oxidesemiconductor film 119 from the outside. Moreover, the use of a nitrideinsulating film with a low hydrogen content as the insulating film 132can suppress change in the electrical characteristics of the transistor103 and the capacitor 105 (MOS capacitor).

The pixel electrode 121 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

For the substrate 150, a material that can be used for the substrate 102can be used.

The light-blocking film 152 is also referred to as a black matrix and isprovided in a liquid crystal display device to suppress leakage of lightof a light source device such as a backlight or suppress contrastreduction due to mixing of colors when color display is performed usinga color filter, for example. A light-blocking film which is generallyused can be used as the light-blocking film 152. A metal and an organicresin including a pigment can be given as examples of a light-blockingmaterial. Alternatively, the light-blocking film 152 may be provided ina region outside the pixel portion 100, such as over the scan linedriver circuit 104 and over the signal line driver circuit 106 (see FIG.1A), as well as over the transistor 103 in the pixel 101.

Note that a coloring film which transmits light with a predeterminedwavelength may be provided across a space between the light-blockingfilms 152 adjacent to each other. Further, an overcoat film may beprovided between the counter electrode 154, and the light-blocking films152 and the coloring film.

For the counter electrode 154, materials that can be used for the pixelelectrode 121 can be used as appropriate.

The alignment films 156 and 158 can be formed using a general-purposematerial such as polyamide.

For the liquid crystal 160, a thermotropic liquid crystal, alow-molecular liquid crystal, a high-molecular liquid crystal, apolymer-dispersed liquid crystal, a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, or the like can be used. Such aliquid crystal material exhibits a cholesteric phase, a smectic phase, acubic phase, a chiral nematic phase, an isotropic phase, or the likedepending on a condition.

Alternatively, a liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used for the liquid crystal 160. Ablue phase is one of liquid crystal phases, which is generated justbefore a cholesteric phase changes into an isotropic phase whiletemperature of cholesteric liquid crystal is raised. Since the bluephase appears only in a narrow temperature range, a liquid crystalcomposition in which a chiral material is mixed is used in order toimprove the temperature range. Note that the alignment film is formedusing an organic resin containing hydrogen, water, or the like, whichmight degrade the electrical characteristics of the transistor in thesemiconductor device of one embodiment of the present invention. In viewof the above, the use of liquid crystal which exhibits a blue phase forthe liquid crystal 160 enables fabrication of the semiconductor deviceof one embodiment of the present invention without an organic resin, sothat the semiconductor device can be highly reliable.

Note that the structure of the liquid crystal element 108 can be changedas appropriate, as follows, in accordance with the display mode of theliquid crystal element 108: the shapes of the pixel electrode 121 andthe counter electrode 154 are changed, or a protrusion referred to as arib is formed.

Further, in the semiconductor device of one embodiment of the presentinvention, a region of the pixel 101, in which the light-blocking film152 is provided, can be reduced or removed in such a manner that apolarization axis of a polarizing member (a polarizing substrate) isprovided to be in parallel to the light-blocking film 152, and thedisplay mode of the semiconductor device is set to a normally-black modein which the liquid crystal element 108 does not transmit light from alight source device such as a backlight with no voltage applied. As aresult, the aperture ratio of the pixel 101 can be improved even in thecase where the size of one pixel is small as in a display device havinga high resolution, where the pixel density is greater than or equal to200 ppi, further greater than or equal to 300 ppi. Note that theaperture ratio can be further increased by using a light-transmittingcapacitor.

<Fabrication Method of Semiconductor Device>

A fabrication method of the above semiconductor device is described withreference to FIGS. 5A and 5B and FIGS. 6A and 6B.

Here, the process for forming the dual-gate transistor 15 is alsodescribed on the assumption that the dual-gate transistor 15 included inthe scan line driver circuit 104 of the semiconductor device is formedover a substrate over which the pixel portion 100 is provided. The gateelectrode 17 a of the dual-gate transistor 15 is formed using the samematerial as the gate electrode 107 a of the transistor 103. The gateinsulating film 12 of the dual-gate transistor 15 is formed using thesame material as the gate insulating film 127 of the transistor 103. Theoxide semiconductor film 11 of the dual-gate transistor 15 is formedusing the same material as the oxide semiconductor film 111 of thetransistor 103. The source electrode 19 a and the drain electrode 13 aof the dual-gate transistor 15 are formed using the same material as thesource electrode 109 a and the drain electrode 113 a of the transistor103. The insulating film 29, the insulating film 31, and the insulatingfilm 32 of the dual-gate transistor 15 are formed using the samematerials as the insulating film 129, the insulating film 131, and theinsulating film 132 of the transistor 103, respectively.

First, the scan line 107 including the gate electrode 107 a and thecapacitor line 115 are formed over the substrate 102. An insulating film126 which will be processed into the gate insulating film 127 later isformed so as to cover the scan line 107 and the capacitor line 115. Theoxide semiconductor film 111 is formed over a region of the insulatingfilm 126, which overlaps with the scan line 107. The oxide semiconductorfilm 119 is formed over the insulating film 126 so as to overlap with aregion where the pixel electrode 121 will be formed later (see FIG. 5A).

Note that by performing the above steps, the gate electrode 17 a, thegate insulating film 12, and the oxide semiconductor film 11 of thedual-gate transistor 15 can be formed (see FIG. 5B).

The scan line 107 and the capacitor line 115 can be formed in such amanner that a conductive film is formed using any of the materials givenabove, a mask is formed over the conductive film, and the conductivefilm is processed using the mask. For the conductive film, any of avariety of film formation methods such as an evaporation method, a CVDmethod, a sputtering method, and a spin coating method can be used. Notethat there is no particular limitation on the thickness of theconductive film, and the thickness of the conductive film can bedetermined in consideration of time needed for the formation, desiredresistivity, or the like. As the mask, a resist mask formed through aphotolithography process can be used. The conductive film can beprocessed by one or both of dry etching and wet etching.

The insulating film 126 can be formed using a material that can be usedfor the gate insulating film 127 by any of a variety of film formationmethods such as a CVD method and a sputtering method.

In the case where gallium oxide is used for the gate insulating film127, the insulating film 126 can be formed by a metal organic chemicalvapor deposition (MOCVD) method.

The oxide semiconductor film 111 and the oxide semiconductor film 119can be formed in such a manner that any of the oxide semiconductor filmsgiven above is formed, a mask is formed over the formed oxidesemiconductor film, and the oxide semiconductor film is processed usingthe mask. The oxide semiconductor film can be formed by a sputteringmethod, a coating method, a pulsed laser deposition method, a laserablation method, or the like. By employing a printing method, the oxidesemiconductor film 111 and the oxide semiconductor film 119 subjected toelement isolation can be formed directly on the insulating film 126. Asa power supply device for generating plasma in the case of forming theoxide semiconductor film by a sputtering method, an RF power supplydevice, an AC power supply device, a DC power supply device, or the likecan be used as appropriate. As a sputtering gas, an atmosphere of a raregas (typically argon), oxygen, or a mixed gas of a rare gas and oxygenis used as appropriate. In the case of the mixed atmosphere of a raregas and oxygen, the proportion of oxygen is preferably higher than thatof a rare gas. Note that the target may be selected as appropriatedepending on the composition of the oxide semiconductor film to beformed. As the mask, a resist mask formed through a photolithographyprocess can be used. The oxide semiconductor film can be processed byone or both of dry etching and wet etching. The etching conditions(e.g., an etching gas or an etching solution, etching time, andtemperature) are set as appropriate depending on the material so thatthe oxide semiconductor film 111 and the oxide semiconductor film 119can be etched to have desired shapes.

Heat treatment is preferably performed after formation of the oxidesemiconductor films 111 and 119 to dehydrate or dehydrogenate the oxidesemiconductor films 111 and 119. The heat treatment is performedtypically at a temperature higher than or equal to 150° C. and lowerthan the strain point of the substrate, preferably higher than or equalto 200° C. and lower than or equal to 450° C., more preferably higherthan or equal to 300° C. and lower than or equal to 450° C. Note thatthe heat treatment may be performed on the oxide semiconductor filmwhich has not been processed into the oxide semiconductor films 111 and119.

In the heat treatment, a heat treatment apparatus is not limited to anelectric furnace; the heat treatment apparatus can be an apparatus thatheats an object using thermal conduction or thermal radiation given by amedium such as a heated gas or the like. For example, a rapid thermalanneal (RTA) apparatus such as a gas rapid thermal anneal (GRTA)apparatus or a lamp rapid thermal anneal (LRTA) apparatus can be used.An LRTA apparatus is an apparatus for heating an object to be processedby radiation of light (an electromagnetic wave) emitted from a lamp suchas a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is less than orequal to 20 ppm, preferably less than or equal to 1 ppm, more preferablyless than or equal to 10 ppb), or a rare gas (e.g., argon or helium).The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gaspreferably does not contain hydrogen, water, and the like.Alternatively, the heat treatment may be performed first under an inertgas atmosphere, and then under an oxygen atmosphere. Note that thetreatment time is 3 minutes to 24 hours.

In the case where a base insulating film is provided between thesubstrate 102, and the scan line 107, the capacitor line 115, and thegate insulating film 127, the base insulating film can be formed using afilm of any of the following: silicon oxide, silicon oxynitride, siliconnitride, silicon nitride oxide, gallium oxide, hafnium oxide, yttriumoxide, aluminum oxide, aluminum oxynitride, and the like. Note that whena film of silicon nitride, gallium oxide, hafnium oxide, yttrium oxide,or aluminum oxide is used as the base insulating film, it is possible tosuppress diffusion of impurities, typically an alkali metal, water, andhydrogen into the oxide semiconductor film 111 from the substrate 102.The base insulating film can be formed by a sputtering method or a CVDmethod.

Next, after the opening 123 reaching the capacitor line 115 is formed inthe insulating film 126 to form the gate insulating film 127, the signalline 109 including the source electrode 109 a, the conductive film 113serving as the drain electrode 113 a, and the conductive film 125 whichelectrically connects the oxide semiconductor film 119 and the capacitorline 115 are formed (see FIG. 5B).

Note that by performing the above steps, the source electrode 19 a, thedrain electrode 13 a, and the conductive film 16 of the dual-gatetransistor 15 can be formed (see FIG. 2B). In addition, the conductivefilm 16 can be formed in contact with the capacitor line 115 by formingthe opening reaching the capacitor line 115 in the gate insulating film12 when the opening 123 is formed.

The opening 123 can be formed so as to expose part of a region of theinsulating film 126, which overlaps with the capacitor line 115, in sucha manner that a mask is formed and the insulating film 126 is processedusing the mask. The formation of the mask and the processing can beperformed in manners similar to those of the scan line 107 and thecapacitor line 115.

The signal line 109, the conductive film 113, and the conductive film125 can be formed in such a manner that a conductive film is formedusing a material that can be used for the signal line 109, theconductive film 113, and the conductive film 125, a mask is formed overthe conductive film, and the conductive film is processed using themask. The formation of the mask and the processing can be performed inmanners similar to those of the scan line 107 and the capacitor line115.

Next, an insulating film 128 is formed over the oxide semiconductor film111, the oxide semiconductor film 119, the signal line 109, theconductive film 113, the conductive film 125, and the gate insulatingfilm 127, an insulating film 130 is formed over the insulating film 128,and an insulating film 133 is formed over the insulating film 130 (seeFIG. 6A). The insulating films 128, 130, and 133 are preferably formedsuccessively, in which case entry of impurities into each interface canbe suppressed.

The insulating film 128 can be formed using a material that can be usedfor the insulating film 129 by any of a variety of film formationmethods such as a CVD method and a sputtering method. The insulatingfilm 130 can be formed using a material that can be used for theinsulating film 131. The insulating film 133 can be formed using amaterial that can be used for the insulating film 132.

In the case where an oxide insulating film which has fewer interfacestates between the oxide semiconductor film 111 and the oxide insulatingfilm is used as the insulating film 129, the insulating film 128 can beformed under the following formation conditions. Here, as the oxideinsulating film, a silicon oxide film or a silicon oxynitride film isformed. As for the formation conditions, the substrate placed in atreatment chamber of a plasma CVD apparatus, which is vacuum-evacuated,is held at a temperature higher than or equal to 180° C. and lower thanor equal to 400° C., preferably higher than or equal to 200° C. andlower than or equal to 370° C., the pressure in the treatment chamber isgreater than or equal to 20 Pa and less than or equal to 250 Pa,preferably greater than or equal to 40 Pa and less than or equal to 200Pa with introduction of a source gas such as a deposition gas containingsilicon and an oxidizing gas into the treatment chamber, andhigh-frequency power is supplied to an electrode provided in thetreatment chamber.

Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, and silane fluoride. Examples of theoxidizing gas include oxygen, ozone, dinitrogen monoxide, and nitrogendioxide.

By setting the ratio of the amount of the oxidizing gas to the amount ofthe deposition gas containing silicon 100 or higher, the amount ofhydrogen contained in the insulating film 128 (the insulating film 129)can be reduced and the number of dangling bonds in the insulating film128 (the insulating film 129) can be reduced. Oxygen transferred fromthe insulating film 130 (the insulating film 131) is trapped by thedangling bonds in the insulating film 128 (the insulating film 129) insome cases; thus, in the case where the dangling bonds in the insulatingfilm 128 (the insulating film 129) are reduced, oxygen in the insulatingfilm 130 (the insulating film 131) can be transferred to at least theoxide semiconductor film 111 efficiently to compensate the oxygenvacancies in the oxide semiconductor film 111. As a result, the amountof hydrogen entering the oxide semiconductor film 111 can be reduced andthe oxygen vacancies in the oxide semiconductor film 111 can be reduced.

In the case where the above oxide insulating film which includes anoxygen excess region or the above oxide insulating film containingoxygen at a higher proportion than oxygen in the stoichiometriccomposition is used as the insulating film 131, the insulating film 130can be formed under the following formation conditions. Here, as theoxide insulating film, a silicon oxide film or a silicon oxynitride filmis formed. As for the formation conditions, the substrate placed in atreatment chamber of the plasma CVD apparatus, which isvacuum-evacuated, is held at a temperature higher than or equal to 180°C. and lower than or equal to 260° C., preferably higher than or equalto 180° C. and lower than or equal to 230° C., the pressure in thetreatment chamber is greater than or equal to 100 Pa and less than orequal to 250 Pa, preferably greater than or equal to 100 Pa and lessthan or equal to 200 Pa with introduction of a source gas into thetreatment chamber, and high-frequency power that is higher than or equalto 0.17 W/cm² and lower than or equal to 0.5 W/cm², preferably higherthan or equal to 0.25 W/cm² and lower than or equal to 0.35 W/cm² issupplied to an electrode provided in the treatment chamber.

As the source gas of the insulating film 130, the source gas that can beused to form the insulating film 128 can be used.

As the formation conditions of the insulating film 130, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; therefore, theoxygen content of the insulating film 130 becomes higher than that inthe stoichiometric composition. In addition, in the film formed at asubstrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film can be released by heat treatment in the later step. Thus, itis possible to form an oxide insulating film containing oxygen at ahigher proportion than the stoichiometric composition and from whichpart of oxygen is released by heating. Moreover, the insulating film 128is provided over the oxide semiconductor film 111. Therefore, in theprocess for forming the insulating film 130, the insulating film 128serves as a protective film of the oxide semiconductor film 111. Thus,even when the insulating film 130 is formed using the high-frequencypower having a high power density, damage to the oxide semiconductorfilm 111 can be suppressed.

By increasing the thickness of the insulating film 130, the amount ofoxygen released by heating can be increased; thus, the insulating film130 is preferably provided thicker than the insulating film 128. Sincethe insulating film 128 is provided, favorable coverage can be achievedeven when the insulating film 130 is formed thick.

The insulating film 132 can be formed by a sputtering method, a CVDmethod, or the like. In the case where a nitride insulating film with alow hydrogen content is used as the insulating film 132, the insulatingfilm 132 can be formed under the following formation conditions. Here,as the nitride insulating film, a silicon nitride film is formed. As forthe formation conditions, the substrate placed in a treatment chamber ofthe plasma CVD apparatus, which is vacuum-evacuated, is held at atemperature higher than or equal to 80° C. and lower than or equal to400° C., preferably higher than or equal to 200° C. and lower than orequal to 370° C., the pressure in the treatment chamber is greater thanor equal to 100 Pa and less than or equal to 250 Pa, preferably greaterthan or equal to 100 Pa and less than or equal to 200 Pa withintroduction of a source gas into the treatment chamber, andhigh-frequency power is supplied to an electrode provided in thetreatment chamber.

As the source gas of the insulating film 132, a deposition gascontaining silicon, nitrogen, and ammonia are preferably used. Typicalexamples of the deposition gas containing silicon include silane,disilane, trisilane, and silane fluoride. Further, the flow rate ofnitrogen is preferably 5 times to 50 times that of ammonia, furtherpreferably 10 times to 50 times that of ammonia. The use of ammonia asthe source gas can promote decomposition of the deposition gascontaining silicon and nitrogen. This is because ammonia is dissociatedby plasma energy or heat energy, and energy generated by thedissociation contributes to decomposition of a bond of the depositiongas molecules containing silicon and a bond of nitrogen molecules. Underthe above conditions, a silicon nitride film which has a low hydrogencontent and can suppress entry of impurities such as hydrogen and waterfrom the outside can be formed.

It is preferable that heat treatment be performed at least afterformation of the insulating film 130 so that oxygen contained in theinsulating film 128 or the insulating film 130 is transferred to atleast the oxide semiconductor film 111 to compensate oxygen vacancies inthe oxide semiconductor film 111. The heat treatment can be performed asappropriate with reference to the details of heat treatment fordehydration or dehydrogenation of the oxide semiconductor film 111 andthe oxide semiconductor film 119.

Next, the opening 117 reaching the conductive film 113 is formed in aregion of the insulating films 128, 130, and 133, which overlaps withthe conductive film 113, so that the insulating films 129, 131, and 132are formed; and then the pixel electrode 121 is formed over theinsulating film 132 and in the opening 117 (see FIG. 6B).

Note that through these steps, the insulating films 29, 31, and 32, theopening 94, and the wiring 92 of the dual-gate transistor 15 can beformed (see FIG. 2B). Through these steps, the capacitor line 115 andthe wiring 92 including the back-gate electrode of the dual-gatetransistor 15 included in the scan line driver circuit 104 areelectrically connected to each other through the conductive film 16.

The opening 117 can be formed in a manner similar to that of the opening123. The pixel electrode 121 is formed in such a manner that aconductive film is formed using any of the materials given above incontact with the conductive film 113 through the opening 117, a mask isformed over the conductive film, and the conductive film is processedusing the mask. The formation of the mask and the processing can beperformed in manners similar to those of the scan line 107 and thecapacitor line 115.

Next, the alignment film 158 is formed over the insulating film 132 andthe pixel electrode 121, and the substrate 150 is formed on thelight-blocking film 152. In addition, the counter electrode 154 isformed so as to cover the light-blocking film 152, and the counterelectrode 154 is formed on the alignment film 156. The liquid crystal160 is provided over the alignment film 158, the substrate 150 isprovided above the substrate 102 so that the alignment film 156 is incontact with the liquid crystal 160, and the substrate 102 and thesubstrate 150 are fixed to each other with a sealant (not illustrated).

The alignment films 156 and 158 can be formed using the above materialby any of a variety of film formation methods such as a spin coatingmethod and a printing method.

The light-blocking film 152 can be formed by a sputtering method usingany of the materials given above and can be processed using a mask. Inthe case where a resin is used, the light-blocking film 152 can beformed through a photolithography process.

The counter electrode 154 can be formed using the material that can beused for the pixel electrode 121 by any of a variety of film formationmethods such as a CVD method and a sputtering method.

The liquid crystal 160 can be directly provided on the alignment film158 by a dispenser method (a dropping method). Alternatively, the liquidcrystal 160 may be injected by using capillary action or the like afterthe substrate 102 and the substrate 150 are attached to each other.Further, the alignment films 156 and 158 are preferably subjected torubbing treatment so that alignment of the liquid crystal 160 is easilyperformed.

Through the above process, the semiconductor device of one embodiment ofthe present invention can be fabricated (see FIG. 4).

Modification Example 1

In the semiconductor device of one embodiment of the present invention,connection of the capacitor line and the semiconductor film(specifically, the oxide semiconductor film) serving as one electrode ofthe capacitor can be changed as appropriate. For example, to improve theaperture ratio, a structure where the semiconductor film is in directcontact with the capacitor line without the conductive film providedtherebetween can be employed.

Note that in the drawings illustrating modification examples below, thesubstrate 150, the light-blocking film 152, the counter electrode 154,the alignment films 156 and 158, and the liquid crystal 160 are omittedfor simplicity of drawing.

This structure is described with reference to FIG. 7 and FIG. 8. FIG. 7is a top view of a pixel 141, and FIG. 8 is a cross-sectional view takenalong dashed-dotted line A1-A2 and dashed-dotted line B1-B2 in FIG. 7.Here, only a capacitor 145 which is different from the capacitor 105described with reference to FIG. 3 and FIG. 4 is described. Thestructure of the pixel 141 in FIG. 7 and FIG. 8 is similar to that inFIG. 3 and FIG. 4, except for the capacitor 145.

In the pixel 141, the oxide semiconductor film 119 serving as oneelectrode of the capacitor 145 is in direct contact with the capacitorline 115 through an opening 143. Unlike in the capacitor 105 in FIG. 4,the oxide semiconductor film 119 is in direct contact with the capacitorline 115 without the conductive film 125 provided therebetween and theconductive film 125 serving as a light-blocking film is not formed, sothat a higher aperture ratio of the pixel 141 can be achieved.

Alternatively, in the semiconductor device of one embodiment of thepresent invention, the structure in which the semiconductor film is indirect contact with the capacitor line may be obtained by employing astructure in which a region where the capacitor line is partly exposedand a region where the substrate is partly exposed are provided insteadof forming an opening in the gate insulating film. FIG. 9 is a top viewof a pixel 101, and FIG. 10 is a cross-sectional view taken alongdashed-dotted line A1-A2 and dashed-dotted line B1-B2 in FIG. 9.

According to FIG. 9 and FIG. 10, in the pixel 101 of this structure,part of the gate insulating film 127 is removed, the capacitor line 115and the substrate 102 are each partly exposed, and the capacitor line115 and the oxide semiconductor film 119 are in direct contact with eachother in these exposed regions. Accordingly, the area in which thecapacitor line 115 and the oxide semiconductor film 119 are in contactwith each other can be increased. Thus, the aperture ratio can beincreased and the capacitor 146 can be brought into conduction easily.

Modification Example 2

In the semiconductor device of one embodiment of the present invention,connection of the capacitor line and the semiconductor film(specifically, the oxide semiconductor film) serving as one electrode ofthe capacitor can be changed as appropriate. For example, to increasethe conductivity of the semiconductor film, the conductive film can beprovided in contact with the semiconductor film along the outerperiphery thereof. This structure is described with reference to FIG. 11and FIGS. 12A and 12B. Here, only a conductive film 167 which isdifferent from the conductive film 125 described with reference to FIG.3 and FIG. 4 is described. FIG. 11 is a top view of a pixel 161, FIG.12A is a cross-sectional view taken along dashed-dotted line A1-A2 anddashed-dotted line B1-B2 in FIG. 11, and FIG. 12B is a cross-sectionalview taken along dashed-dotted line D1-D2 in FIG. 11.

In the pixel 161, the conductive film 167 is in contact with the oxidesemiconductor film 119 along the outer periphery thereof and is incontact with the capacitor line 115 through the opening 123 (see FIG.11). The conductive film 167 is formed in the same formation process asthe signal line 109 including the source electrode 109 a of thetransistor 103 and the conductive film 113 serving as the drainelectrode 113 a (not illustrated) of the transistor 103 and thus mayhave a light-blocking property; for this reason, the conductive film 167is preferably formed into a loop shape.

As illustrated in FIGS. 12A and 12B, in the pixel 161 of this structure,the conductive film 167 is provided so as to cover end portions of theoxide semiconductor film 119 of the capacitor 105. The structure of thepixel 161 in FIG. 11 and FIGS. 12A and 12B is similar to that in FIG. 3and FIG. 4, except for the conductive film 167.

In the structure illustrated in FIG. 11 and FIGS. 12A and 12B, theconductive film 167 is formed into a loop shape; however, a portion ofthe conductive film 167, which is in contact with the oxidesemiconductor film 119, does not have to be entirely electricallyconnected to the capacitor line 115. In other words, a conductive filmformed in the same formation process as the conductive film 167 may beprovided in contact with the oxide semiconductor film 119 so as to beseparated from the conductive film 167.

Modification Example 3

In the semiconductor device of one embodiment of the present invention,connection of the capacitor line and the semiconductor film(specifically, the oxide semiconductor film) serving as one electrode ofthe capacitor can be changed as appropriate. Specific example of thisstructure is described with reference to FIG. 13 and FIG. 14. Here, onlyan oxide semiconductor film 177 and a capacitor line 175 which aredifferent from the oxide semiconductor film 119 and the capacitor line115 described with reference to FIG. 3 and FIG. 4 is described. FIG. 13is a top view of a pixel 171 where the capacitor line 175 extends in adirection parallel to the signal line 109. The signal line 109 and thecapacitor line 175 are electrically connected to the signal line drivercircuit 106 (see FIG. 1A).

A capacitor 173 is connected to the capacitor line 175 which extends ina direction parallel to the signal line 109. The capacitor 173 includesthe light-transmitting oxide semiconductor film 177 formed by utilizingthe process for forming the oxide semiconductor film 111, thelight-transmitting pixel electrode 121, and, as a dielectric film, thelight-transmitting insulating films (not illustrated in FIG. 13) whichare included in the transistor 103. That is, the capacitor 173 transmitslight.

Next, FIG. 14 is a cross-sectional view taken along dashed-dotted lineA1-A2 and dashed-dotted line B1-B2 in FIG. 13.

In the capacitor 173, the oxide semiconductor film 177 serves as one ofa pair of electrodes, the pixel electrode 121 serves as the other of thepair of electrodes, and the insulating films 129, 131, and 132 serve asa dielectric film provided between the pair of electrodes.

The capacitor line 175 can be formed by utilizing the process forforming the signal line 109 and the conductive film 113. When thecapacitor line 175 is provided in contact with the oxide semiconductorfilm 177, the area where the oxide semiconductor film 177 and thecapacitor line 175 are in contact with each other can be increased. Theoxide semiconductor film 177 can easily serve as one electrode of thecapacitor 173.

Further, in the pixel 171 illustrated in FIG. 13, the length of thepixel 171 in the direction in which the signal line 109 extends islonger than the length of the pixel 171 in the direction in which thescan line 107 extends. However, as in a pixel 172 illustrated in FIG.15, the following structure may be employed: the length of the pixel 172in the direction in which the scan line 107 extends is longer than thelength of the pixel 172 in which the signal line 109 extends, and acapacitor line 176 extends in a direction parallel to the signal line109. Note that the signal line 109 and the capacitor line 176 areelectrically connected to the signal line driver circuit 106 (see FIG.1A).

A capacitor 174 is connected to the capacitor line 176 which extends ina direction parallel to the signal line 109. The capacitor 174 includesa light-transmitting oxide semiconductor film 178 formed by utilizingthe process for forming the oxide semiconductor film 111, thelight-transmitting pixel electrode 121, and, as a dielectric film, thelight-transmitting insulating films (not illustrated in FIG. 15) whichare included in the transistor 103. That is, the capacitor 174 transmitslight.

Next, FIG. 16 is a cross-sectional view taken along dashed-dotted lineA1-A2 and dashed-dotted line B1-B2 in FIG. 15.

In the capacitor 174, the oxide semiconductor film 178 serves as one ofa pair of electrodes, the pixel electrode 121 serves as the other of thepair of electrodes, and the insulating films 129, 131, and 132 serve asa dielectric film provided between the pair of electrodes.

The capacitor line 176 can be formed by utilizing the process forforming the signal line 109 and the conductive film 113. When thecapacitor line 176 is provided in contact with the oxide semiconductorfilm 178, the area where the oxide semiconductor film 178 and thecapacitor line 176 are in contact with each other can be increased.Further, in the pixel 172, the length of the pixel 172 in the directionin which the scan line 107 extends is longer than the length of thepixel 172 in the direction in which the signal line 109 extends;therefore, the area in which the pixel electrode 121 and the capacitorline 176 are overlapped with each other can be reduced and the apertureratio can be improved as compared to the pixel 171 illustrated in FIG.13.

Modification Example 4

To reduce parasitic capacitance generated between the pixel electrode121 and the conductive film 113 and parasitic capacitance generatedbetween the pixel electrode 121 and the conductive film 125 in the abovepixels 101, 141, 161, 171, and 172, an organic insulating film 134 canbe provided in a region where the parasitic capacitance is generated asillustrated in a cross-sectional view in FIG. 17. The structure in FIG.17 is the same as that in FIG. 4 except for the organic insulating film134. Here, only the organic insulating film 134 not included in thestructure in FIG. 4 is described.

For the organic insulating film 134, a photosensitive organic resin or anon-photosensitive organic resin can be used; for example, an acrylicresin, a benzocyclobutene resin, an epoxy resin, a siloxane-based resin,or the like can be used. Alternatively, polyamide can be used for theorganic insulating film 134.

The organic insulating film 134 can be formed in such manner that anorganic resin film is formed using any of the materials given above andprocessed. When a photosensitive organic resin is used for the organicinsulating film 134, a resist mask is unnecessary in formation of theorganic insulating film 134 and thus a process can be simplified.Therefore, a formation method of the organic insulating film is notparticularly limited and can be selected as appropriate in accordancewith a material which is used. For example, a CVD method, a sputteringmethod, spin coating, dipping, spray coating, a droplet discharge method(such as an inkjet method), screen printing, offset printing, or thelike can be used.

In general, an organic resin contains much hydrogen and water; thus,when an organic resin is provided over the transistor 103 (inparticular, the oxide semiconductor film 111), hydrogen and watercontained in the organic resin diffuses into the transistor 103 (inparticular, the oxide semiconductor film 111) and might degrade theelectrical characteristics of the transistor 103. For this reason, it ispreferable that the organic insulating film 134 be not provided at leastover a portion of the insulating film 132, which overlaps with the oxidesemiconductor film 111. In other words, it is preferable that a regionof the organic resin film, which is over a region overlapping with atleast the oxide semiconductor film 111, be removed.

FIG. 18 is a top plan view of the pixel 101 illustrated in FIG. 17. Thecross-sectional view in FIG. 17 corresponds to cross sections takenalong dashed-dotted line A1-A2, dashed-dotted line B1-B2, anddashed-dotted line C1-C2 in FIG. 18. In FIG. 18, the organic insulatingfilm 134 is not illustrated for simplification; however, a regionindicated by dashed-two dotted lines is a region where the organicinsulating film 134 is not provided.

Modification Example 5

In the semiconductor device of one embodiment of the present invention,one electrode of the capacitor and the capacitor line can be formedusing a semiconductor film (specifically, an oxide semiconductor film).Specific example is described with reference to FIG. 19. Here, only anoxide semiconductor film 198 which is different from the oxidesemiconductor film 119 and the capacitor line 115 described withreference to FIG. 3 is described. FIG. 19 is a top view of a pixel 196where the oxide semiconductor film 198 serving as one electrode of acapacitor 197 and the capacitor line is provided in the pixel 196. Theoxide semiconductor film 198 has a region which extends in a directionparallel to the signal line 109 and the region serves as the capacitorline. In the oxide semiconductor film 198, a region overlapping with thepixel electrode 121 serves as one electrode of the capacitor 197. Notethat in the oxide semiconductor film 198, the maximum width of adepletion layer gets extremely larger by applying an electric field;therefore, the oxide semiconductor film 198 is brought into conduction.

The oxide semiconductor film 198 can be formed by utilizing the processfor forming the oxide semiconductor film 111 of the transistor 103provided in the pixel 196.

One oxide semiconductor film can be provided as the oxide semiconductorfilm 198 for the pixels 196 so as to overlap with the scan lines 107. Inother words, a continuous oxide semiconductor film can be provided forthe pixels 196 in one row.

In the case where a continuous oxide semiconductor film is provided asthe oxide semiconductor film 198 for the pixels 196 in one row, theoxide semiconductor film 198 overlaps with the scan lines 107. For thisreason, the oxide semiconductor film 198 in some cases does notsufficiently serve as the capacitor line and one electrode of thecapacitor 197 due to an effect of a change in the potential of the scanline 107. Thus, as illustrated in FIG. 19, the oxide semiconductor films198 are preferably separated from each other between the pixels 196, andthe separated oxide semiconductor films are electrically connected toeach other through a conductive film 199 which can be formed byutilizing the process for forming the signal line 109 and the conductivefilm 113.

In FIG. 19, a region of the oxide semiconductor film 198, which servesas the capacitor line, extends in a direction parallel to the signalline 109; however, the region which serves as the capacitor line mayextend in a direction parallel to the scan line 107. In the case wherethe region of the oxide semiconductor film 198, which serves as thecapacitor line, extends in a direction parallel to the scan line 107, itis necessary that the oxide semiconductor film 111 and the oxidesemiconductor film 198 be electrically insulated from the signal line109 and the conductive film 113 by providing an insulating film betweenthe oxide semiconductor film 111 and the oxide semiconductor film 198,and the signal line 109 and the conductive film 113, in the transistor103 and the capacitor 197.

As described above, when a light-transmitting oxide semiconductor filmis provided for one electrode of a capacitor provided in a pixel and acapacitor line as in the pixel 196, the pixel can have a higher apertureratio.

Modification Example 6

In the semiconductor device of one embodiment of the present invention,the structure of the capacitor line can be changed as appropriate. Thisstructure is described with reference to FIG. 20. In FIG. 20, unlike thecapacitor line 115 described with reference to FIGS. 2A and 2B, acapacitor line is provided between adjacent two pixels.

FIG. 20 is a top view of a pixel 401_1 and a pixel 401_2 adjacent toeach other in the direction in which a signal line 409 extends.

A scan line 407_1 and a scan line 407_2 are provided so as to extend inparallel to each other in the direction substantially perpendicular tothe signal line 409. A capacitor line 415 is provided between the scanlines 407_1 and 407_2 so as to be in parallel to the scan lines 407_1and 407_2. The capacitor line 415 is connected to a capacitor 405_1provided in the pixel 401_1 and a capacitor 405_2 provided in the pixel401_2. Top surface shape and the positions of components of the pixel401_1 and those of the pixel 401_2 are symmetric with respect to thecapacitor line 415.

The pixel 401_1 is provided with a transistor 403_1 and the capacitor405_1 connected to the transistor 403_1.

The transistor 403_1 is provided in a region where the scan line 407_1and the signal line 409 intersect with each other. The transistor 403_1includes at least a semiconductor film 411_1 including a channelformation region, a gate electrode, a gate insulating film (notillustrated in FIG. 20), a source electrode, and a drain electrode. Aregion of the scan line 407_1, which overlaps with the semiconductorfilm 411_1, serves as the gate electrode of the transistor 403_1. Aregion of the signal line 409, which overlaps with the semiconductorfilm 411_1, serves as the source electrode of the transistor 403_1. Aregion of a conductive film 413_1, which overlaps with the semiconductorfilm 411_1, serves as the drain electrode of the transistor 403_1. Theconductive film 413_1 and a pixel electrode 421_1 are connected to eachother through an opening 417_1.

The capacitor 405_1 is electrically connected to the capacitor line 415through a conductive film 425 provided in and over an opening 423. Thecapacitor 405_1 includes a semiconductor film 419_1 formed with alight-transmitting oxide semiconductor, the light-transmitting pixelelectrode 421_1, and, as a dielectric film, a light-transmittinginsulating film (not illustrated in FIG. 20) which is included in thetransistor 403_1. That is, the capacitor 405_1 transmits light.

The pixel 401_2 is provided with a transistor 403_2 and the capacitor405_2 connected to the transistor 4032.

The transistor 403_2 is provided in a region where the scan line 407_2and the signal line 409 intersect with each other. The transistor 403_2includes at least a semiconductor film 411_2 including a channelformation region, a gate electrode, a gate insulating film (notillustrated in FIG. 20), a source electrode, and a drain electrode. Aregion of the scan line 407_2, which overlaps with the semiconductorfilm 411_2, serves as the gate electrode of the transistor 403_2. Aregion of the signal line 409, which overlaps with the semiconductorfilm 411_2, serves as the source electrode of the transistor 403_2. Aregion of a conductive film 413_2, which overlaps with the semiconductorfilm 411_2, serves as the drain electrode of the transistor 403_2. Theconductive film 413_2 and a pixel electrode 421_2 are connected to eachother through an opening 417_2.

The capacitor 405_2 is electrically connected to the capacitor line 415through the conductive film 425 provided in and over the opening 423 ina manner similar to that of the capacitor 405_1. The capacitor 405_2includes a semiconductor film 419_2 formed with a light-transmittingoxide semiconductor, the light-transmitting pixel electrode 421_2, and,as a dielectric film, a light-transmitting insulating film (notillustrated in FIG. 20) which is included in the transistor 403_2. Thatis, the capacitor 405_2 transmits light.

Cross-sectional structures of the transistors 403_1 and 403_2 and thecapacitors 405_1 and 405_2 are similar to those of the transistor 103and the capacitor 105 illustrated in FIG. 3 and thus descriptionsthereof are omitted here.

In a structure seen from above, a capacitor line is provided betweenadjacent two pixels so that capacitors included in the pixels and thecapacitor line are connected, whereby the number of capacitor lines canbe reduced. As a result, the aperture ratio of the pixel can be high ascompared with the case of a structure where each pixel is provided witha capacitor line. For example, in the semiconductor device of oneembodiment of the present invention, the aperture ratio of the pixel canbe 61.7% in the pixel layout illustrated in FIG. 20 with the size of onepixel of 28 μm (H)×84 μm (V) and the pixel density of 302 ppi.

Modification Example 7

In the above pixels 101, 141, 161, 171, 172, 196, 401_1, and 401_2, theshapes of the transistors provided in the pixels are not limited to theshapes of the transistors illustrated in FIG. 3 and FIG. 4 and can bechanged as appropriate. For example, in a pixel 151, a transistor 169may be as follows: a source electrode 109 a (not illustrated) includedin the signal line 109 has a U shape (or a C shape, asquare-bracket-like shape, or a horseshoe shape), which partly surrounds(see FIG. 21) the conductive film 113 serving as a drain electrode 113 a(not illustrated). With such a shape, an enough channel width can beensured even when the area of the transistor is small, and accordingly,the amount of on-state current of the transistor can be increased. Thestructure of the pixel 151 in FIG. 21 is similar to that in FIG. 3,except for the transistor 169.

Modification Example 8

As the transistor in each of the above pixels 101, 141, 161, 171, 172,196, 4011, and 4012, a transistor in which an oxide semiconductor filmis formed between a gate insulating film, and a signal line including asource electrode and a conductive film serving as a drain electrode isused. As illustrated in FIG. 22, as such a transistor, a transistor 190in which an oxide semiconductor film 195 is formed between theinsulating film 129, and a signal line 191 including a source electrode191 a and a conductive film 193 serving as a drain electrode 193 a canbe used. The structure in FIG. 22 is the same as that in FIG. 4 exceptfor the position of the oxide semiconductor film 195.

In the transistor 190 illustrated in FIG. 22, the signal line 191 andthe conductive film 193 are formed and then the oxide semiconductor film195 is formed. Thus, a surface of the oxide semiconductor film 195 isnot exposed to an etchant or an etching gas used in a formation processof the signal line 191 and the conductive film 193, so that impuritiesbetween the oxide semiconductor film 195 and the insulating film 129 canbe reduced. Accordingly, a leakage current flowing between the sourceelectrode 191 a and the drain electrode 193 a of the transistor 190 canbe reduced.

Modification Example 9

In the above pixels 101, 141, 161, 171, 172, 196, 401_1, and 401_2, achannel-etched transistor is used as the transistor. Instead of thetransistor, a channel protective transistor 183 can be used asillustrated in FIG. 23. The structure in FIG. 23 is the same as that inFIG. 4 except that a channel protective film 182 is provided between theoxide semiconductor film 111, and the signal line 109 including thesource electrode 109 a and the conductive film 113 serving as the drainelectrode 113 a.

In the channel protective transistor 183 in FIG. 23, the channelprotective film 182 is formed over the oxide semiconductor film 111 andthen the signal line 109 and the conductive film 113 are formed. Thechannel protective film 182 can be formed using the material of theinsulating film 129 of the transistor 103, in which case it is notnecessary to additionally provide an insulating film corresponding tothe insulating film 129 of the transistor 103 in the channel protectivetransistor 183. Therefore, a surface of the oxide semiconductor film 111is not exposed to an etchant or an etching gas used in a formationprocess of the signal line 191 and the conductive film 193, so thatimpurities between the oxide semiconductor film 111 and the channelprotective film 182 can be reduced. Accordingly, a leakage currentflowing between the source electrode 109 a and the drain electrode 113 aof the channel protective transistor 183 can be reduced. Further, it ispossible to suppress, with the channel protective film 182, damage tothe oxide semiconductor film 111 (in particular, the channel formationregion), which is caused by processing performed at the formation of thesignal line 109 and the conductive film 113.

Modification Example 10

As the transistor in each of the above pixels 101, 141, 161, 171, 172,196, 401_1, and 401_2, a transistor including one gate electrode isshown. Alternatively, instead of such a transistor, a transistor 185including a conductive film 187 which faces the gate electrode includedin the scan line 107 with the oxide semiconductor film 111 providedtherebetween can be used as illustrated in FIG. 24.

The transistor 185 is a dual gate transistor which is a transistor inwhich the conductive film 187 is formed over the insulating film 132included in any of the transistors 103, 169, and 190 described in thisembodiment. The conductive film 187 can be called a back-gate electrode.The conductive film 187 overlaps with at least a channel formationregion of the oxide semiconductor film 111. By providing the conductivefilm 187 so as to overlap with the channel formation region of the oxidesemiconductor film 111, the amount of change of the threshold voltage ofthe transistor 185 between before and after a reliability test (e.g.,Bias Temperature (BT) stress test) can be further reduced. Further, thepotential of the conductive film 187 is favorably a common potential, aGND potential, or an arbitrary potential. Alternatively, the conductivefilm 187 may be in a floating state. By controlling the potential of theconductive film 187, the threshold voltage of the transistor 185 can becontrolled. Alternatively, the gate electrode 107 a and the conductivefilm 187 may be electrically connected to each other so as to have thesame potential. Further, when the conductive film 187 is provided, aninfluence of a change in ambient electric field on the oxidesemiconductor film 111 can be reduced, leading to an improvement inreliability of the transistor.

The conductive film 187 can be formed using a material and a methodsimilar to those of the scan line 107, the signal line 109, the pixelelectrode 121, or the like.

As described above, in a semiconductor device including a drivercircuit, the operation speed of the driver circuit can be increasedwhile power consumption can be reduced in such a manner that a dual-gatetransistor is used as a transistor included in the driver circuit and aback-gate electrode of the dual-gate transistor is electricallyconnected to a capacitor line electrically connected to a capacitor;accordingly, a semiconductor device having excellent display quality canbe obtained.

In addition, the use of the semiconductor film formed in the sameformation process as the semiconductor film (specifically, the oxidesemiconductor film) of the transistor, for one electrode of thecapacitor, allows fabrication of a semiconductor device including thecapacitor whose charge capacity is increased while the aperture ratio isimproved. As a result, the semiconductor device can have an excellentdisplay quality.

Further, oxygen vacancies and impurities such as hydrogen and nitrogenin the semiconductor film (specifically, the oxide semiconductor film)of the transistor are reduced, so that the semiconductor device of oneembodiment of the present invention has favorable electricalcharacteristics.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

Embodiment 2

In this embodiment, a semiconductor device of one embodiment of thepresent invention, which has a structure different from those describedin the above embodiment, will be described with reference to drawings.Note that in this embodiment, a liquid crystal display device isdescribed as an example of the semiconductor device of one embodiment ofthe present invention. In the semiconductor device described in thisembodiment, the structure of a capacitor is different from that of thecapacitor in the above embodiment. The above embodiment can be referredto for components in the semiconductor device in this embodiment, whichare similar to those of the semiconductor device in the aboveembodiment.

<Structure of Semiconductor Device>

FIG. 25 is a top view of a pixel 201 in this embodiment. The pixel 201illustrated in FIG. 25 is different from the pixel 101 illustrated inFIG. 3 in that an insulating film 229 and an insulating film 231 whichare not illustrated are not provided in a region indicated by analternate long and tow short dashed line. Thus, a capacitor 205 in thepixel 201 in FIG. 25 includes the oxide semiconductor film 119 servingas one electrode, a pixel electrode 221 serving as the other electrode,and an insulating film 232 (not illustrated) serving as a dielectricfilm.

Next, FIG. 26 is a cross-sectional view taken along dashed-dotted lineA1-A2 and dashed-dotted line B1-B2 in FIG. 25.

The cross-sectional structure of the pixel 201 of this embodiment is asfollows. Over the substrate 102, the scan line 107 including the gateelectrode 107 a and the capacitor line 115 which is on the same surfaceas the scan line 107 are provided. The gate insulating film 127 isprovided over the scan line 107 and the capacitor line 115. The oxidesemiconductor film 111 is provided over a region of the gate insulatingfilm 127, which overlaps with the scan line 107, and the oxidesemiconductor film 119 is provided over the gate insulating film 127.The signal line 109 including the source electrode 109 a and theconductive film 113 serving as the drain electrode 113 a are providedover the oxide semiconductor film 111 and the gate insulating film 127.The opening 123 reaching the capacitor line 115 is provided in the gateinsulating film 127, and the conductive film 125 is provided in and overthe opening 123 and over the gate insulating film 127 and the oxidesemiconductor film 119. The insulating films 229, 231, and 232 whicheach serve as a protective insulating film of the transistor 103 isprovided over the gate insulating film 127, the signal line 109, theoxide semiconductor film 111, the conductive film 113, the conductivefilm 125, and the oxide semiconductor film 119. The opening 117 reachingthe conductive film 113 is provided in the insulating films 229, 231,and 232, and the pixel electrode 221 is provided in and over the opening117 and over the insulating film 232. Note that a base insulating filmmay be provided between the substrate 102, and the scan line 107, thecapacitor line 115, and the gate insulating film 127.

The insulating film 229 is similar to the insulating film 129 describedin Embodiment 1. The insulating film 231 is similar to the insulatingfilm 131 described in Embodiment 1. The insulating film 232 is similarto the insulating film 132 described in Embodiment 1. The pixelelectrode 221 is similar to the pixel electrode 121 described inEmbodiment 1.

When the insulating film 232 serves as a dielectric film between theoxide semiconductor film 119 serving as one electrode and the pixelelectrode 221 serving as the other electrode as in the capacitor 205 inthis embodiment, the thickness of the dielectric film can be thinnerthan that of the dielectric film of the capacitor 105 in Embodiment 1.Thus, the capacitor 205 in this embodiment can have larger chargecapacity than the capacitor 105 in Embodiment 1.

The insulating film 232 is preferably a nitride insulating film which issimilar to the insulating film 132 in Embodiment 1. The insulating film232 is in contact with the oxide semiconductor film 119, so thatnitrogen and/or hydrogen contained in the nitride insulating film can betransferred to the oxide semiconductor film 119 and thus the oxidesemiconductor film 119 can have a higher conductivity. Further, when theinsulating film 232 is formed using a nitride insulating film and issubjected to heat treatment while it is in contact with the oxidesemiconductor film 119, nitrogen and/or hydrogen contained in thenitride insulating film can be transferred to the oxide semiconductorfilm 119. Accordingly, the oxide semiconductor film 119 becomes ann-type oxide semiconductor film with increased conductivity. Theconductivity of the oxide semiconductor film 119 is higher than that ofthe oxide semiconductor film 111; thus, it can be said that the oxidesemiconductor film 119 is a film having conductivity.

As described above, in the semiconductor device of this embodiment, theoxide semiconductor film 119 includes a region having a higherconductivity than the oxide semiconductor film 111. At least a region ofthe oxide semiconductor film 119, which is in contact with theinsulating film 232, has a higher conductivity than a region of theoxide semiconductor film 111, which is in contact with the insulatingfilm 229.

Note that it is preferable that the oxide semiconductor film 119 have ahigher hydrogen concentration than the oxide semiconductor film 111. Inthe oxide semiconductor film 119, the hydrogen concentration measured bysecondary ion mass spectrometry (SIMS) is greater than or equal to8×10¹⁹ atoms/cm³, preferably greater than or equal to 1×10²⁰ atoms/cm³,more preferably greater than or equal to 5×10²⁰ atoms/cm³. In the oxidesemiconductor film 111, the hydrogen concentration measured by SIMS isless than 5×10¹⁹ atoms/cm³, preferably less than 5×10¹⁸ atoms/cm³, morepreferably less than or equal to 1×10¹⁸ atoms/cm³, still more preferablyless than or equal to 5×10¹⁷ atoms/cm³, further preferably less than orequal to 1×10¹⁶ atoms/cm³.

The resistivity of the oxide semiconductor film 119 is lower than thatof the oxide semiconductor film 111. The resistivity of the oxidesemiconductor film 119 is preferably greater than or equal to 1×10⁻⁸times and less than or equal to 1×10⁻¹ times the resistivity of theoxide semiconductor film 111. The resistivity of the oxide semiconductorfilm 119 is typically greater than or equal to 1×10⁻³ Ωcm and less than1×10⁴ Ωcm, preferably greater than or equal to 1×10⁻³ Ωcm and less than1×10⁻¹ Ωcm.

<Fabrication Method of Semiconductor Device>

Next, a fabrication method of a semiconductor device of this embodimentis described with reference to FIGS. 27A and 27B and FIGS. 28A and 28B.

First, the scan line 107 including the gate electrode 107 a and thecapacitor line 115 are formed over the substrate 102. An insulating filmwhich will be processed into the gate insulating film 127 is formed overthe substrate 102, the scan line 107, and the capacitor line 115. Theoxide semiconductor film 111 and the oxide semiconductor film 119 areformed over the insulating film. The opening 123 reaching the capacitorline 115 is formed in the insulating film to form the gate insulatingfilm 127 and then the signal line 109 including the source electrode 109a, the conductive film 113, and the conductive film 125 are formed. Theinsulating film 128 is formed over the gate insulating film 127, thesignal line 109, the conductive film 113, the conductive film 125, andthe oxide semiconductor film 119. The insulating film 130 is formed overthe insulating film 128 (see FIG. 27A). Note that the above steps can beperformed with reference to Embodiment 1.

Next, a mask is formed over a region of the insulating film 130, whichoverlaps with at least the oxide semiconductor film 119. Processing isperformed to form an insulating film 228 and an insulating film 230 withthe use of the mask and expose the oxide semiconductor film 119. Aninsulating film 233 is formed over the exposed region and the insulatingfilm 130 (see FIG. 2713). As the mask, a resist mask formed through aphotolithography process can be used, and the processing can beperformed by one or both of dry etching and wet etching. The insulatingfilm 233 is similar to the insulating film 133 described inEmbodiment 1. Note that heat treatment may be performed while theinsulating film 233 is in contact with the oxide semiconductor film 119,for example, after formation of the insulating film 233. The above stepscan be performed with reference to Embodiment 1.

When the insulating film 233 is formed using a nitride insulating filmby a plasma CVD method or a sputtering method, the oxide semiconductorfilm 119 is exposed to plasma and oxygen vacancies are generated in theoxide semiconductor film 119. Moreover, when the oxide semiconductorfilm 119 is in contact with the insulating film 233 formed using anitride insulating film, nitrogen and/or hydrogen are/is transferredfrom the insulating film 233 to the oxide semiconductor film 119. Due toentry of hydrogen contained in the insulating film 233 into an oxygenvacancy, an electron serving as a carrier is generated. Alternatively,when the insulating film 232 is formed using a nitride insulating filmand is subjected to heat treatment while it is in contact with the oxidesemiconductor film 119, nitrogen and/or hydrogen contained in thenitride insulating film can be transferred to the oxide semiconductorfilm 119. Accordingly, the oxide semiconductor film 119 has increasedconductivity to be n-type. Further, the oxide semiconductor film 119becomes a light-transmitting conductive film which includes a metaloxide film having conductor characteristics. Note that the conductivityof the oxide semiconductor film 119 is higher than that of the oxidesemiconductor film 111.

Next, the opening 117 reaching the conductive film 113 is formed in theinsulating films 228, 230, and 233 to form the insulating films 229,231, and 232 (see FIG. 28A). Then, the pixel electrode 221 in contactwith the conductive film 113 through the opening 117 is formed (see FIG.28B). The above steps can be performed with reference to Embodiment 1.

Through the above process, the semiconductor device in this embodimentcan be fabricated.

Modification Example

In the semiconductor device of one embodiment of the present invention,the structure of the capacitor can be changed as appropriate. A specificexample of the structure is described with reference to FIG. 29. Here,only a capacitor 245 which is different from the capacitor 205 describedwith reference to FIG. 3 and FIG. 4 is described.

A gate insulating film 218 has a stacked-layer structure of aninsulating film 226 formed using a nitride insulating film and aninsulating film 227 formed using an oxide insulating film and only theinsulating film 226 is provided in a region where at least the oxidesemiconductor film 119 is provided. With such a structure, the nitrideinsulating film for forming the insulating film 226 is in contact withthe bottom surface of the oxide semiconductor film 119, so that theoxide semiconductor film 119 can have a higher conductivity (see FIG.29). FIG. 29 is a cross-sectional view, and FIG. 3 can be referred tofor the top view corresponding to FIG. 29. In this case, a dielectricfilm of the capacitor 105 is the insulating films 129, 131, and 132. Asthe insulating films 226 and 227, the insulating films which can be usedas the gate insulating film 127 can be used as appropriate, and theinsulating film 227 may be formed using an insulating film similar tothe insulating film 132. Further, to obtain this structure, theinsulating film 227 is processed as appropriate with reference toEmbodiment 1.

In the structure illustrated in FIG. 29, the top surface of the oxidesemiconductor film 119 may be in contact with the insulating film 132.That is, regions of the insulating films 129 and 131 in FIG. 29, whichare in contact with the oxide semiconductor film 119, may be removed. Inthat case, a dielectric film of the capacitor 105 is the insulating film132. When the top and bottom surfaces of the oxide semiconductor film119 are in contact with the nitride insulating film, the oxidesemiconductor film 119 can have a higher conductivity more efficientlyand sufficiently than the oxide semiconductor film 119 which is incontact with only one of surfaces of the nitride insulating film.

As described above, according to one embodiment of the presentinvention, in a semiconductor device including a driver circuit, theoperation speed of the driver circuit can be increased while powerconsumption can be reduced in such a manner that a dual-gate transistoris used as a transistor included in the driver circuit and a back-gateelectrode of the dual-gate transistor is electrically connected to acapacitor line electrically connected to a capacitor; accordingly, asemiconductor device having excellent display quality can be obtained.

In addition, the use of the semiconductor film formed in the sameformation process as the semiconductor film (specifically, the oxidesemiconductor film) of the transistor, for one electrode of thecapacitor, allows fabrication of a semiconductor device including thecapacitor whose charge capacity is increased while the aperture ratio isimproved. Further, the semiconductor device can have an excellentdisplay quality by improving the aperture ratio.

Further, oxygen vacancies and impurities such as hydrogen and nitrogenin the semiconductor film (specifically, the oxide semiconductor film)of the transistor are reduced, so that the semiconductor device of oneembodiment of the present invention has favorable electricalcharacteristics.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and themodification examples in the other embodiments.

Embodiment 3

In this embodiment, a semiconductor device of one embodiment of thepresent invention, which has a structure different from those describedin the above embodiment, will be described with reference to drawings.Note that in this embodiment, a liquid crystal display device isdescribed as an example of the semiconductor device of one embodiment ofthe present invention. In the semiconductor device described in thisembodiment, the structure of a capacitor is different from that of thecapacitor in the above embodiment. The above embodiment can be referredto for components in the semiconductor device in this embodiment, whichare similar to those of the semiconductor device in the aboveembodiment.

<Structure of Semiconductor Device>

Next, a specific example of the structure of a pixel 301 provided in apixel portion of the liquid crystal display device described in thisembodiment is described. FIG. 30 is a top view of the pixel 301. Thepixel 301 illustrated in FIG. 30 includes the capacitor 305, and thecapacitor 305 is provided in a region of the pixel 301, which issurrounded by the capacitor line 115 and the signal line 109. Thecapacitor 305 is electrically connected to the capacitor line 115through the conductive film 125 provided in and over the opening 123.The capacitor 305 includes an oxide semiconductor film 319 which has ahigher conductivity than the oxide semiconductor film 111 and has alight-transmitting property, the light-transmitting pixel electrode 121,and, as a dielectric film, the light-transmitting insulating films (notillustrated in FIG. 30) which are included in the transistor 103. Thatis, the capacitor 305 transmits light.

The conductivity of the oxide semiconductor film 319 is higher than orequal to 10 S/cm and lower than or equal to 1000 S/cm, preferably higherthan or equal to 100 S/cm and lower than or equal to 1000 S/cm.

The oxide semiconductor film 319 has such a high conductivity and thuscan sufficiently serve as the electrode of the capacitor. That is, thecapacitor 305 can be formed large (in a large area) in the pixel 301.For this reason, the semiconductor device can have charge capacityincreased while the aperture ratio is improved. As a result, thesemiconductor device can have an excellent display quality.

Next, FIG. 31 is a cross-sectional view taken along dashed-dotted lineA1-A2 and dashed-dotted line B1-B2 in FIG. 30.

The cross-sectional structure of the pixel 301 is as follows. Over thesubstrate 102, the scan line 107 including the gate electrode 107 a isprovided. The gate insulating film 127 is provided over the scan line107. The oxide semiconductor film 111 is provided over a region of thegate insulating film 127, which overlaps with the scan line 107, and theoxide semiconductor film 319 is provided over another region of the gateinsulating film 127. The signal line 109 including the source electrode109 a and the conductive film 113 serving as the drain electrode 113 aare provided over the oxide semiconductor film 111 and the gateinsulating film 127. In addition, the capacitor line 115 is providedover the gate insulating film 127 and the oxide semiconductor film 319.The insulating films 129, 131, and 132 which each serve as a protectiveinsulating film of the transistor 103 is provided over the gateinsulating film 127, the signal line 109, the oxide semiconductor film111, the conductive film 113, the oxide semiconductor film 319, and thecapacitor line 115. The opening 117 reaching the conductive film 113 isprovided in the insulating films 129, 131, and 132, and the pixelelectrode 121 is provided in and over the opening 117 and over theinsulating film 132. Note that a base insulating film may be providedbetween the substrate 102, and the scan line 107 and the gate insulatingfilm 127.

In the capacitor 305 in this example, the oxide semiconductor film 319which has a higher conductivity than the oxide semiconductor film 111serves as one of a pair of electrodes, the pixel electrode 121 serves asthe other of the pair of electrodes, and the insulating films 129, 131,and 132 serve as a dielectric film provided between the pair ofelectrodes.

For the oxide semiconductor film 319, an oxide semiconductor that can beused for the oxide semiconductor film 111 can be used. The oxidesemiconductor film 319 can be formed concurrently with the oxidesemiconductor film 111 and thus contains a metal element of an oxidesemiconductor included in the oxide semiconductor film 111. Further, theoxide semiconductor film 319 preferably has a higher conductivity thanthe oxide semiconductor film 111 and thus preferably contains an element(dopant) which increases the conductivity. Specifically, the oxidesemiconductor film 319 contains one or more selected from boron,nitrogen, fluorine, aluminum, phosphorus, arsenic, indium, tin,antimony, and a rare gas element as the dopant. The concentration of adopant contained in the oxide semiconductor film 319 is preferablygreater than or equal to 1×10¹⁹ atoms/cm³ and less than or equal to1×10²² atoms/cm³, in which case the conductivity of the oxidesemiconductor film 319 can be greater than or equal to 10 S/cm and lessthan or equal to 1000 S/cm, preferably greater than or equal to 100 S/cmand less than or equal to 1000 S/cm, so that the oxide semiconductorfilm 319 can sufficiently serve as one electrode of the capacitor 305.The oxide semiconductor film 319 has a region with a higher conductivitythan that of the oxide semiconductor film 111. In this embodiment, atleast a region of the oxide semiconductor film 319, which is in contactwith the insulating film 132, has a higher conductivity than a region ofthe oxide semiconductor film 111, which is in contact with theinsulating film 129. Further, the oxide semiconductor film 319 is n-typeand has a high conductivity because of including the above element(dopant); therefore, the oxide semiconductor film 319 can be called aconductive film.

<Fabrication Method of Semiconductor Device>

Next, a fabrication method of a semiconductor device of this embodimentis described with reference to FIGS. 32A and 32B and FIGS. 33A and 33B.

First, the scan line 107 including the gate electrode 107 a and thecapacitor line 115 are formed over the substrate 102. An insulating filmwhich will be processed into the gate insulating film 127 is formed overthe substrate 102, the scan line 107, and the capacitor line. The oxidesemiconductor film 111 and the oxide semiconductor film 119 are formedover the insulating film (see FIG. 32A). Note that the above steps canbe performed with reference to Embodiment 1.

Next, a dopant is added to the oxide semiconductor film 119 to form theoxide semiconductor film 319, the opening 123 reaching the capacitorline 115 is formed in the insulating film 126 to form the gateinsulating film 127, and then the signal line 109 including the sourceelectrode 109 a, the conductive film 113 serving as the drain electrode113 a, and the conductive film 125 which electrically connects the oxidesemiconductor film 319 and the capacitor line 115 are formed (see FIG.32B).

A method of adding a dopant to the oxide semiconductor film 119 is asfollows: a mask is provided in a region except the oxide semiconductorfilm 119 and one or more dopants selected from boron, nitrogen,fluorine, aluminum, phosphorus, arsenic, indium, tin, antimony, and arare gas element is added to the oxide semiconductor film 119 by an ionimplantation method, an ion doping method, or the like. Alternatively,the oxide semiconductor film 119 may be exposed to plasma containing thedopant to add the dopant to the oxide semiconductor film 119, instead ofemploying an ion implantation method or an ion doping method. Note thatheat treatment may be performed after the dopant is added to the oxidesemiconductor film 119. The heat treatment can be performed asappropriate with reference to the details of the heat treatment fordehydration or dehydrogenation of the oxide semiconductor film 111 andthe oxide semiconductor film 119 in Embodiment 1.

The step of adding the dopant may be performed after formation of thesignal line 109, the conductive film 113, and the conductive film 125,in which case the dopant is not added to regions of the oxidesemiconductor film 319, which are in contact with the signal line 109,the conductive film 113, and the conductive film 125.

Next, the insulating film 128 is formed over the gate insulating film127, the signal line 109, the oxide semiconductor film 111, theconductive film 113, the conductive film 125, and the oxidesemiconductor film 319. The insulating film 130 is formed over theinsulating film 128, and the insulating film 133 is formed over theinsulating film 130 (see FIG. 33A). The above steps can be performedwith reference to Embodiment 1.

Next, the opening 117 reaching the conductive film 113 is formed in theinsulating films 128, 130, and 133 to form the insulating films 129,131, and 132 (see FIG. 33B). The pixel electrode 121 in contact with theconductive film 113 through the opening 117 is formed (see FIG. 31). Theabove steps can be performed with reference to Embodiment 1.

Through the above process, the semiconductor device in this embodimentcan be fabricated.

As described above, according to one embodiment of the presentinvention, in a semiconductor device including a driver circuit, theoperation speed of the driver circuit can be increased while powerconsumption can be reduced in such a manner that a dual-gate transistoris used as a transistor included in the driver circuit and a back-gateelectrode of the dual-gate transistor is electrically connected to acapacitor line electrically connected to a capacitor; accordingly, asemiconductor device having excellent display quality can be obtained.

In addition, the use of the semiconductor film formed in the sameformation process as the semiconductor film (specifically, the oxidesemiconductor film) of the transistor, for one electrode of thecapacitor, allows fabrication of a semiconductor device including thecapacitor whose charge capacity is increased while the aperture ratio isimproved. Further, the semiconductor device can have an excellentdisplay quality by improving the aperture ratio.

Further, oxygen vacancies and impurities such as hydrogen and nitrogenin the semiconductor film (specifically, the oxide semiconductor film)of the transistor are reduced, so that the semiconductor device of oneembodiment of the present invention has favorable electricalcharacteristics.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

Embodiment 4

In this embodiment, one embodiment which can be applied to an oxidesemiconductor film, which is a semiconductor film, in the transistor andthe capacitor included in the semiconductor device described in theabove embodiment will be described.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, apolycrystalline oxide semiconductor film, a microcrystalline oxidesemiconductor film, an amorphous oxide semiconductor film, and the like.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

In this specification, a term “parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −10° and lessthan or equal to 10°, and accordingly also includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Inaddition, a term “perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 80° and less than orequal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits inside a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. Note thatwhen a plurality of crystal parts included in the CAAC-OS film areconnected to each other, one large crystal region is formed in somecases. For example, a crystal region with an area of 2500 nm² or more, 5μm² or more, or 1000 μm² or more is observed in some cases in the planTEM image.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the x-axis of the crystal is aligned witha direction parallel to a normal vector of a formation surface or anormal vector of a top surface. Thus, for example, in the case where ashape of the CAAC-OS film is changed by etching or the like, the c-axismight not be necessarily parallel to a normal vector of a formationsurface or a normal vector of a top surface of the CAAC-OS film.

Further, distribution of c-axis aligned crystal parts in the CAAC-OSfilm is not necessarily uniform. For example, in the case where crystalgrowth leading to the crystal parts of the CAAC-OS film occurs from thevicinity of the top surface of the film, the proportion of the c-axisaligned crystal parts in the vicinity of the top surface is higher thanthat in the vicinity of the formation surface in some cases. Further,when an impurity is added to the CAAC-OS film, a region to which theimpurity is added is altered, and the proportion of the c-axis alignedcrystal parts in the CAAC-OS film varies depending on regions, in somecases.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appears at around 31° and a peak of 2θ do not appear ataround 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, the oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has small change in electrical characteristics andhigh reliability. Electric charge trapped by the carrier traps in theoxide semiconductor film takes a long time to be released, and mightbehave like fixed electric charge. Thus, the transistor including theoxide semiconductor film having high impurity concentration and a highdensity of defect states has unstable electrical characteristics in somecases.

In a transistor using the CAAC-OS film, change in the electricalcharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small.

Next, a microcrystalline oxide semiconductor film is described.

In a TEM image of the microcrystalline oxide semiconductor film, crystalparts sometimes cannot be found clearly. In most cases, the size of acrystal part in the microcrystalline oxide semiconductor film is greaterthan or equal to 1 nm and less than or equal to 100 nm, or greater thanor equal to 1 nm and less than or equal to 10 nm. A microcrystal with asize greater than or equal to 1 nm and less than or equal to 10 nm, or asize greater than or equal to 1 nm and less than or equal to 3 nm isspecifically referred to as nanocrystal (nc). An oxide semiconductorfilm including nanocrystal is referred to as an nc-OS (nanocrystallineoxide semiconductor) film. In a TEM image, a crystal grain cannot befound clearly in the nc-OS film in some cases.

In the nc-OS film, a microscopic region (e.g., a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. However, there is noregularity of crystal orientation between different crystal parts in thenc-OS film; thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor depending on an analysis method. Forexample, when the nc-OS film is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Further, a halo pattern is shown in aselected-area electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter (e.g., larger than orequal to 50 nm) larger than that of a crystal part. Meanwhile, spots areshown in a nanobeam electron diffraction pattern of the nc-OS filmobtained by using an electron beam having a probe diameter (e.g., largerthan or equal to 1 nm and smaller than or equal to 30 nm) close to, orsmaller than or equal to that of a crystal part. Further, in a nanobeamelectron diffraction pattern of the nc-OS film, regions with highluminance in a circular (ring) pattern are shown in some cases. Also ina nanobeam electron diffraction pattern of the nc-OS film, a pluralityof spots are shown in a ring-like region in some cases.

Since the nc-OS film is an oxide semiconductor film having moreregularity than the amorphous oxide semiconductor film, the nc-OS filmhas a lower density of defect states than the amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

For example, there are three methods for forming a CAAC-OS film.

The first method is to form an oxide semiconductor film at a temperaturehigher than or equal to 100° C. and lower than or equal to 450° C.,whereby crystal parts in which the c-axes are aligned in the directionparallel to a normal vector of a surface on which the oxidesemiconductor film is formed or a normal vector of a surface of theoxide semiconductor film are formed in the oxide semiconductor film.

The second method is to form an oxide semiconductor film with a smallthickness and then heat it at a temperature higher than or equal to 200°C. and lower than or equal to 700° C., whereby crystal parts in whichthe c-axes are aligned in the direction parallel to a normal vector of asurface on which the oxide semiconductor film is formed or a normalvector of a surface of the oxide semiconductor film are formed in theoxide semiconductor film.

The third method is to form a first oxide semiconductor film with asmall thickness, then heat it at a temperature higher than or equal to200° C. and lower than or equal to 700° C., and form a second oxidesemiconductor film, whereby crystal parts in which the c-axes arealigned in the direction parallel to a normal vector of a surface onwhich the oxide semiconductor film is formed or a normal vector of asurface of the oxide semiconductor film are formed in the oxidesemiconductor film.

In a transistor using the CAAC-OS film for an oxide semiconductor film,change in the electrical characteristics of the transistor due toirradiation with visible light or ultraviolet light is small. Thus, thetransistor using the CAAC-OS film as the oxide semiconductor film hashigh reliability.

For example, it is preferable that the CAAC-OS film is formed by asputtering method with a polycrystalline oxide semiconductor sputteringtarget. When ions collide with the sputtering target, a crystal regionincluded in the sputtering target may be separated from the target alongan a-b plane; in other words, a sputtered particle having a planeparallel to an a-b plane (flat-plate-like sputtered particle orpellet-like sputtered particle) may flake off from the sputteringtarget. In that case, the flat-plate-like sputtered particle or thepellet-like sputtered particle reaches a surface on which the CAAC-OSfilm is formed while maintaining its crystal state, whereby the CAAC-OSfilm can be deposited.

For the deposition of the CAAC-OS film, the following conditions arepreferably employed.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is lower than or equal to −80° C.,preferably lower than or equal to −100° C. is used.

By increasing the heating temperature of the surface on which theCAAC-OS film is formed (e.g., the substrate heating temperature) duringthe deposition, migration of a sputtered particle is likely to occurafter the sputtered particle reaches the surface on which the CAAC-OSfilm is formed. Specifically, the temperature of the surface on whichthe CAAC-OS film is formed during the deposition is higher than or equalto 100° C. and lower than or equal to 740° C., preferably higher than orequal to 150° C. and lower than or equal to 500° C. By increasing thetemperature of the surface on which the CAAC-OS film is formed duringthe deposition, when the flat-plate-like or pellet-like sputteredparticle reaches the surface on which the CAAC-OS film is formed,migration occurs on the surface on which the CAAC-OS film is formed, sothat a flat plane of the sputtered particle is attached to the surfaceon which the CAAC-OS film is formed.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is higher than or equal to 30 vol %, preferably 100 vol%.

As an example of the sputtering target, an In—Ga—Zn—O compound target isdescribed below.

The polycrystalline In—Ga—Zn-based metal oxide target is made by mixingInO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in a predeterminedmolar ratio, applying pressure, and performing heat treatment at atemperature higher than or equal to 1000° C. and lower than or equal to1500° C. This pressure treatment may be performed while cooling isperformed or may be performed while heating is performed. Note that X,Y, and Z are each a given positive number. Here, the predetermined molarratio of InO_(X) powder to GaO_(Y) powder and ZnO_(Z) powder is, forexample, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds ofpowder and the molar ratio for mixing powder may be determined asappropriate depending on the desired sputtering target.

Further, the oxide semiconductor film may have a structure in which aplurality of oxide semiconductor films are stacked. For example, theoxide semiconductor film may have a stacked-layer structure of a firstoxide semiconductor film and a second oxide semiconductor film which areformed using metal oxides with different atomic ratios. For example, thefirst oxide semiconductor film may be formed using one of an oxidecontaining two kinds of metals, an oxide containing three kinds ofmetals, and an oxide containing four kinds of metals, and the secondoxide semiconductor film may be formed using one of the above which isdifferent from the one used for the first oxide semiconductor film.

Alternatively, the oxide semiconductor film may have a two-layerstructure of the first oxide semiconductor film and the second oxidesemiconductor film, in which the constituent elements thereof are madethe same and the atomic ratios of the constituent elements of the firstoxide semiconductor film and the second oxide semiconductor film aremade different. For example, the first oxide semiconductor film maycontain In, Ga, and Zn at an atomic ratio of 3:1:2, and the second oxidesemiconductor film may contain In, Ga, and Zn at an atomic ratio of1:1:1. Alternatively, the first oxide semiconductor film may contain In,Ga, and Zn at an atomic ratio of 2:1:3, and the second oxidesemiconductor film may contain In, Ga, and Zn at an atomic ratio of1:3:2. Note that a proportion of each atom in the atomic ratio of theoxide semiconductor film varies within a range of ±20% as an error.

At this time, in one of the first oxide semiconductor film and thesecond oxide semiconductor film, which is closer to the gate electrode(on the channel side), the atomic ratio of In to Ga preferably satisfiesthe relation In Ga. In the other oxide semiconductor film, which isfarther from the gate electrode (on the back channel side), the atomicratio of In to Ga preferably satisfies the relation In≦Ga. With astacked-layer structure of these oxide semiconductor films, a transistorhaving high field-effect mobility can be formed. On the other hand, theatomic ratio of In to Ga in the oxide semiconductor film closer to thegate electrode (the oxide semiconductor film on the channel side)satisfies the relation In<Ga and the atomic ratio of In to Ga in theoxide semiconductor film on the back channel side satisfies the relationIn Ga, whereby the amount of change of the threshold voltage of atransistor due to change over time or a reliability test can be reduced.

The first oxide semiconductor film containing In, Ga, and Zn at anatomic ratio of 1:3:2 can be formed by a sputtering method using anoxide target with an atomic ratio of 1:3:2 under the conditions wherethe substrate temperature is room temperature and a sputtering gas isargon or a mixed gas of argon and oxygen. The second oxide semiconductorfilm containing In, Ga, and Zn at an atomic ratio of 3:1:2 can be formedby a sputtering method using an oxide target with an atomic ratio of3:1:2 in a manner similar to that of the first oxide semiconductor film.

Further, the oxide semiconductor film may have a three-layer structureof a first oxide semiconductor film, a second oxide semiconductor film,and a third oxide semiconductor film, in which the constituent elementsthereof are made the same and the atomic ratios of the constituentelements of the first oxide semiconductor film, the second oxidesemiconductor film, and the third oxide semiconductor film are madedifferent. The case where the oxide semiconductor film has a three-layerstructure is described with reference to FIG. 34.

In a transistor 297 illustrated in FIG. 34, a first oxide semiconductorfilm 299 a, a second oxide semiconductor film 299 b, and a third oxidesemiconductor film 299 c are stacked in this order from the gateinsulating film 127 side. As a material of the first oxide semiconductorfilm 299 a and the third oxide semiconductor film 299 c, a materialrepresented by InM1_(x)Zn_(y)O_(z) (x≧1, y>1, z>0, M1=Ga, Hf, or thelike) is used. Note that in the case where a material of each of thefirst oxide semiconductor film 299 a and the third oxide semiconductorfilm 299 c contains Ga, a material containing a high proportion of Ga,specifically, a material which can be represented by InM1_(x)Zn_(y)O_(z)where x is larger than 10 is unsuitable because powder might begenerated in deposition. Note that the structure of the transistor 297is the same as those of the transistors described in the aboveembodiments (e.g., the transistor 103 in Embodiment 1) except that thefirst oxide semiconductor film 299 a, the second oxide semiconductorfilm 299 b, and the third oxide semiconductor film 299 c are included.

As a material of the second oxide semiconductor film 299 b, a materialwhich can be represented by InM2_(x)Zn_(y)O_(z) (x≧1, y≧x, z>0, M2=Ga,Sn, or the like) is used.

Materials of the first oxide semiconductor film 299 a, the second oxidesemiconductor film 299 b, and the third oxide semiconductor film 299 care selected as appropriate so that a well structure is formed in whichthe conduction band of the second oxide semiconductor film 299 b isdeeper from the vacuum level than the conduction bands of the firstoxide semiconductor film 299 a and the third oxide semiconductor film299 c.

As described in Embodiment 1, in the oxide semiconductor film, siliconor carbon, which belongs to Group 14, causes generation of an electronserving as a carrier, leading to an increase in carrier density.Therefore, silicon or carbon contained in an oxide semiconductor filmmakes it n-type. Thus, the concentration of silicon contained in oxidesemiconductor films and the concentration of carbon contained in oxidesemiconductor films are each less than or equal to 3×10¹⁸/cm³,preferably less than or equal to 3×10¹⁷/cm³. It is particularlypreferable to employ a structure where the first oxide semiconductorfilm 299 a and the third oxide semiconductor film 299 c sandwich orsurround the second oxide semiconductor film 299 b serving as a carrierpath so that a large number of Group 14 elements do not enter the secondoxide semiconductor film 299 b. That is, the first oxide semiconductorfilm 299 a and the third oxide semiconductor film 299 c can also becalled barrier films which prevent Group 14 elements such as silicon andcarbon from entering the second oxide semiconductor film 299 b.

For example, the first oxide semiconductor film 299 a may contain In,Ga, and Zn at an atomic ratio of 1:3:2, the second oxide semiconductorfilm 299 b may contain In, Ga, and Zn at an atomic ratio of 3:1:2, andthe third oxide semiconductor film 299 c may contain In, Ga, and Zn atan atomic ratio of 1:1:1. Note that the third oxide semiconductor film299 c can be formed by a sputtering method using an oxide targetcontaining In, Ga, and Zn at an atomic ratio of 1:1:1.

Alternatively, a three-later structure may be employed in which thefirst oxide semiconductor film 299 a contains In, Ga, and Zn at anatomic ratio of 1:3:2, the second oxide semiconductor film 299 bcontains In, Ga, and Zn at an atomic ratio of 1:1:1 or 1:3:2, and thethird oxide semiconductor film 299 c contains In, Ga, and Zn at anatomic ratio of 1:3:2.

Since the constituent elements of the first oxide semiconductor film 299a, the second oxide semiconductor film 299 b, and the third oxidesemiconductor film 299 c are the same, the second oxide semiconductorfilm 299 b has fewer defect states (trap levels) at the interface withthe first oxide semiconductor film 299 a. Specifically, the defectstates (trap levels) are fewer than those at the interface between thegate insulating film 127 and the first oxide semiconductor film 299 a.For this reason, when the oxide semiconductor films are stacked in theabove manner, the amount of change of the threshold voltage of atransistor due to change over time or a reliability test can be reduced.

Further, when materials of the first oxide semiconductor film 299 a, thesecond oxide semiconductor film 299 b, and the third oxide semiconductorfilm 299 c are selected as appropriate so that a well structure isformed in which the conduction band of the second oxide semiconductorfilm 299 b is deeper from the vacuum level than the conduction bands ofthe first oxide semiconductor film 299 a and the third oxidesemiconductor film 299 c, the field-effect mobility of the transistorcan be increased and the amount of change of the threshold voltage ofthe transistor due to change over time or a reliability test can bereduced.

Further, the first oxide semiconductor film 299 a, the second oxidesemiconductor film 299 b, and the third oxide semiconductor film 299 cmay be formed using oxide semiconductors having different crystallinity.That is, the oxide semiconductor film may be formed using a combinationof any of a single crystal oxide semiconductor, a polycrystalline oxidesemiconductor, an amorphous oxide semiconductor, and a CAAC-OS, asappropriate. When an amorphous oxide semiconductor is applied to any oneof the first oxide semiconductor film 299 a, the second oxidesemiconductor film 299 b, and the third oxide semiconductor film 299 c,internal stress or external stress of the oxide semiconductor film canbe relieved, fluctuation in characteristics of the transistors can bereduced, and the amount of change of the threshold voltage of thetransistor due to change over time or a reliability test can be reduced.

At least the second oxide semiconductor film 299 b, which can serve as achannel formation region, is preferably a CAAC-OS film. An oxidesemiconductor film on the back channel side, in this embodiment, thethird oxide semiconductor film 299 c is preferably an amorphous oxidesemiconductor film or a CAAC-OS film. With such a structure, the amountof change of the threshold voltage of a transistor due to change overtime or a reliability test can be reduced.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

Embodiment 5

A semiconductor device (also referred to as a display device) having adisplay function can be fabricated using the transistor and thecapacitor examples of which are shown in the above embodiments.Moreover, some or all of driver circuits which include the transistorcan be formed over a substrate where a pixel portion is formed, wherebya system-on-panel can be obtained. In this embodiment, an example of adisplay device using the transistor examples of which are shown in theabove embodiments is described with reference to FIGS. 35A to 35C, FIGS.36A and 36B, and FIGS. 37A to 37C. FIGS. 36A and 36B are cross-sectionalviews illustrating cross-sectional structures taken along dashed-dottedline M-N in FIG. 35B. Note that in FIGS. 36A and 36B, only part of thestructure of a pixel portion is illustrated.

In FIG. 35A, a sealant 905 is provided so as to surround a pixel portion902 provided over a first substrate 901, and the pixel portion 902 issealed with a second substrate 906. In FIG. 35A, a signal line drivercircuit 903 and a scan line driver circuit 904 are each formed using asingle crystal semiconductor or a polycrystalline semiconductor over asubstrate prepared separately, and mounted in a region different fromthe region surrounded by the sealant 905 over the first substrate 901.Further, various signals and potentials are supplied to the signal linedriver circuit 903, the scan line driver circuit 904, and the pixelportion 902 from flexible printed circuits (FPCs) 918 a and 918 b.

In FIGS. 35B and 35C, the sealant 905 is provided so as to surround thepixel portion 902 and the scan line driver circuit 904 which areprovided over the first substrate 901. The second substrate 906 isprovided over the pixel portion 902 and the scan line driver circuit904. Thus, the pixel portion 902 and the scan line driver circuit 904are sealed together with a display element by the first substrate 901,the sealant 905, and the second substrate 906. In FIGS. 35B and 35C, asignal line driver circuit 903 which is formed using a single crystalsemiconductor or a polycrystalline semiconductor over a substrateseparately prepared is mounted in a region different from the regionsurrounded by the sealant 905 over the first substrate 901. In FIGS. 35Band 35C, various signals and potentials are supplied to the signal linedriver circuit 903, the scan line driver circuit 904, and the pixelportion 902 from an FPC 918.

Although FIGS. 35B and 35C each illustrate an example in which thesignal line driver circuit 903 is formed separately and mounted on thefirst substrate 901, one embodiment of the present invention is notlimited to this structure. The scan line driver circuit may beseparately formed and then mounted, or only part of the signal linedriver circuit or part of the scan line driver circuit may be separatelyformed and then mounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method, or the like canbe employed. FIG. 35A illustrates an example in which the signal linedriver circuit 903 and the scan line driver circuit 904 are mounted by aCOG method. FIG. 35B illustrates an example in which the signal linedriver circuit 903 is mounted by a COG method. FIG. 35C illustrates anexample in which the signal line driver circuit 903 is mounted by a TABmethod.

The display device includes in its category a panel in which a displayelement is sealed and a module in which an IC including a controller orthe like is mounted on the panel.

A display device in this specification refers to an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device also includes the following modulesin its category: a module to which a connector such as an FPC or a TCPis attached; a module having a TCP at the tip of which a printed wiringboard is provided; and a module in which an integrated circuit (IC) isdirectly mounted on a display element by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors and any of thetransistors which are described in the above embodiments can be applied.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. A light-emitting element includes, in its scope,an element whose luminance is controlled by current or voltage, andspecifically includes an inorganic electroluminescent (EL) element, anorganic EL element, and the like. Further, a display medium whosecontrast is changed by an electric effect, such as electronic ink, canbe used. An example of a liquid crystal display device using a liquidcrystal element as the display element is illustrated in FIGS. 36A and36B.

The liquid crystal display device illustrated in FIG. 36A is a liquidcrystal display device of a vertical electric field mode. A liquidcrystal display device includes a connection terminal electrode 915 anda terminal electrode 916. The connection terminal electrode 915 and theterminal electrode 916 are electrically connected to a terminal includedin the FPC 918 through an anisotropic conductive agent 919.

The connection terminal electrode 915 is formed using the sameconductive film as a first electrode 930, and the terminal electrode 916is formed using the same conductive film as a source electrode and adrain electrode of each of a transistor 910 and a transistor 911.

Each of the pixel portion 902 and the scan line driver circuit 904 whichare provided over the first substrate 901 includes a plurality oftransistors. FIGS. 36A and 36B illustrate the transistor 910 included inthe pixel portion 902 and the transistor 911 included in the scan linedriver circuit 904. An insulating film 924 corresponding to theinsulating films 129, 131, and 132 described in Embodiment 1 is providedover the transistors 910 and 911. Note that an insulating film 923 is aninsulating film serving as a base film.

In this embodiment, any of the transistors described in the aboveembodiments can be applied to the transistors 910 and 911. A capacitor926 is formed using an oxide semiconductor film 927, the insulating film924, and the first electrode 930. The oxide semiconductor film 927 isconnected to a capacitor line 929 through an electrode film 928. Theelectrode film 928 is formed using the same conductive film as thesource electrode and the drain electrode of each of the transistors 910and 911. The capacitor line 929 is formed using the same conductive filmas a gate electrode of each of the transistors 910 and 911. Although thecapacitor described in Embodiment 1 is illustrated as the capacitor 926here, any of the capacitors in the other embodiments may be used asappropriate.

Moreover, an example in which a conductive film 917 is provided over theinsulating film 924 so as to overlap with a channel formation region ofthe oxide semiconductor film of the transistor 911 included in the scanline driver circuit is illustrated. That is, the transistor 911 is thedual-gate transistor described in Embodiment 1. Although notillustrated, the conductive film 917 is electrically connected to thecapacitor line 929. In this embodiment, the conductive film 917 isformed using the same conductive film as the first electrode 930. Withsuch a structure, the structure which is configured to control thepotential of the conductive film 917 can be omitted. By providing theconductive film 917 so as to overlap with the channel formation regionof the oxide semiconductor film, the amount of change of the thresholdvoltage of the transistor 911 between before and after a reliabilitytest can be further reduced. Further, the operation speed of thetransistor 911 can be increased; thus, the operation speed of the drivercircuit can be improved. The conductive film 917 may have the samepotential as or a potential different from that of the gate electrode ofthe transistor 911, and the conductive film 917 can serve as a secondgate electrode (back-gate electrode). The potential difference betweenthe conductive film 917 and the source electrode of the transistor 911may be 0 V. With the above structure, both an increase of the operationspeed and a reduction of power consumption of the display device can beachieved.

In addition, the conductive film 917 has a function of blocking anexternal electric field. In other words, the conductive film 917 has afunction of preventing an external electric field (particularly, afunction of preventing static electricity) from affecting the inside (acircuit portion including the transistor). Such a blocking function ofthe conductive film 917 can suppress change in the electricalcharacteristics of the transistor due to the influence of an externalelectric field such as static electricity. Further, the thresholdvoltage of the transistor can be controlled. Note that although thetransistors included in the scan line driver circuit are illustrated inFIGS. 36A and 36B, in a manner similar to that of the transistor 911, atransistor included in the signal line driver circuit may have astructure in which a conductive film is provided over the insulatingfilm 924 so as to overlap with a channel formation region of the oxidesemiconductor film.

In the display panel, the transistor 910 included in the pixel portion902 is electrically connected to a display element. There is noparticular limitation on the kind of the display element as long asdisplay can be performed, and various kinds of display elements can beused.

A liquid crystal element 913 which is a display element includes thefirst electrode 930, a second electrode 931, and a liquid crystal 908.Note that an insulating film 932 and an insulating film 933 which serveas an alignment film are provided so that the liquid crystal 908 isprovided therebetween. The second electrode 931 is provided on thesecond substrate 906 side. The second electrode 931 overlaps with thefirst electrode 930 with the liquid crystal 908 provided therebetween.For the liquid crystal element 913, the description of the liquidcrystal element 108 in Embodiment 1 can be referred to. The firstelectrode 930 corresponds to the pixel electrode 121 in Embodiment 1,the second electrode 931 corresponds to the counter electrode 154 inEmbodiment 1, the liquid crystal 908 corresponds to the liquid crystal160 in Embodiment 1, the insulating film 932 corresponds to thealignment film 158 in Embodiment 1, and the insulating film 933corresponds to the alignment film 156 in Embodiment 1.

The first electrode and the second electrode (each of which are alsoreferred to as a pixel electrode, a common electrode, a counterelectrode, or the like) for applying voltage to the display element canhave light-transmitting properties or light-reflecting properties, whichdepends on the direction in which light is extracted, the position wherethe electrodes are provided, and the pattern structure of theelectrodes.

The first electrode 930 and the second electrode 931 can be formedusing, as appropriate, a material similar to that of the pixel electrode121 and the counter electrode 154 of Embodiment 1.

A spacer 935 is a columnar spacer obtained by selective etching of aninsulating film and is provided in order to control the distance betweenthe first electrode 930 and the second electrode 931 (a cell gap).Alternatively, a spherical spacer may be used.

The first substrate 901 and the second substrate 906 are fixed in placeby a sealant 925. As the sealant 925, an organic resin such as athermosetting resin or a photocurable resin can be used. In addition,the sealant 925 is in contact with the insulating film 924. Note thatthe sealant 925 corresponds to the sealant 905 in FIGS. 35A to 35C.

In the liquid crystal display device, a black matrix (a light-blockingfilm); an optical member (an optical substrate) such as a polarizingmember, a retardation member, or an anti-reflection member; and the likeare provided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a light source device such as a backlight or a side light maybe used as a light source.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protection circuit is preferably formed using anonlinear element.

Next, a liquid crystal display device of a transverse electric fieldmode is described with reference to FIG. 36B. FIG. 36B illustrates aliquid crystal display device of a fringe field switching (FFS) mode,which is one of transverse electric field modes. The structure of theliquid crystal display device of a transverse electric field mode, whichis different from the structure of the liquid crystal display device ofa vertical electric field mode illustrated in FIG. 36A, is described.

In the liquid crystal display device illustrated in FIG. 36B, theconnection terminal electrode 915 is formed using the same conductivefilm as a first electrode 940, and the terminal electrode 916 is formedusing the same conductive film as the source electrode and the drainelectrode of each of the transistors 910 and 911.

In addition, a liquid crystal element 943 includes the first electrode940, a second electrode 941, and the liquid crystal 908 which are formedover the insulating film 924. The liquid crystal element 943 can have,as appropriate, the structure of the liquid crystal element 108 ofEmbodiment 1. The first electrode 940 can be formed using, asappropriate, the material of the first electrode 930 illustrated in FIG.36A. Further, the planar shape of the first electrode 940 is a comb-likeshape, a staircase-like shape, a ladder-like shape, or the like. Thesecond electrode 941 serves as a common electrode and can be formed in amanner similar to that of the oxide semiconductor film 119 ofEmbodiment 1. The insulating film 924 is provided between the firstelectrode 940 and the second electrode 941. In the liquid crystaldisplay device illustrated in FIG. 36B, the capacitor includes the firstelectrode 940 and the second electrode, which are a pair of electrodes,and the insulating film 924 which serves as a dielectric film.

The second electrode 941 is connected to a capacitor line 946 through aconductive film 945. The conductive film 945 is formed using the sameconductive film as the source electrode and the drain electrode of eachof the transistors 910 and 911. The capacitor line 946 is formed usingthe same conductive film as the gate electrode of each of thetransistors 910 and 911. Although the description is made using thecapacitor described in Embodiment 1 as the liquid crystal element 943here, any of the capacitors described in the other embodiments can beused as appropriate.

FIGS. 37A to 37C illustrate an example of the liquid crystal displaydevice in FIG. 36A in which a common connection portion (pad portion)for being electrically connected to the second electrode 931 provided onthe second substrate 906 is formed over the first substrate 901.

The common connection portion is provided in a position overlapping withthe sealant for bonding the first substrate 901 and the second substrate906, and is electrically connected to the second electrode 931 throughconductive particles contained in the sealant. Alternatively, the commonconnection portion is provided in a position not overlapping with thesealant (except for the pixel portion) and a paste including conductiveparticles is provided separately from the sealant so as to overlap withthe common connection portion, whereby the common connection portion iselectrically connected to the second electrode 931.

FIG. 37A is a cross-sectional view of the common connection portiontaken along line in the top view in FIG. 37B.

A common potential line 975 is provided over a gate insulating film 922and is formed using the same material and through the same steps as asource electrode 971 or a drain electrode 973 of the transistor 910illustrated in FIGS. 36A and 36B.

Further, the common potential line 975 is covered with the insulatingfilm 924, and the insulating film 924 has a plurality of openings at aposition overlapping with the common potential line 975. These openingsare formed through the same steps as a contact hole which connects thefirst electrode 930 and one of the source electrode 971 and the drainelectrode 973 of the transistor 910.

Further, the common potential line 975 is connected to a commonelectrode 977 through the openings. The common electrode 977 is providedover the insulating film 924 and is formed using the same material andthrough the same steps as the connection terminal electrode 915 and thefirst electrode 930 in the pixel portion.

In this manner, the common connection portion can be formed through thesame fabrication process as the switching element in the pixel portion902.

The common electrode 977 is an electrode in contact with the conductiveparticles contained in the sealant, and is electrically connected to thesecond electrode 931 of the second substrate 906.

Alternatively, as illustrated in FIG. 37C, a common potential line 985may be formed using the same material and through the same steps as thegate electrode of the transistor 910.

In the common connection portion illustrated in FIG. 37C, the commonpotential line 985 is provided under the gate insulating film 922 andthe insulating film 924; and the gate insulating film 922 and theinsulating film 924 have a plurality of openings at a positionoverlapping with the common potential line 985. These openings areformed by etching the insulating film 924 through the same steps as acontact hole which connects the first electrode 930 and one of thesource electrode 971 and the drain electrode 973 of the transistor 910,and then by further selectively etching the gate insulating film 922.

Further, the common potential line 985 is connected to a commonelectrode 987 through the openings. The common electrode 987 is providedover the insulating film 924 and is formed using the same material andthrough the same steps as the connection terminal electrode 915 and thefirst electrode 930 in the pixel portion.

As described above, in a semiconductor device including a drivercircuit, the operation speed of the driver circuit can be increasedwhile power consumption can be reduced in such a manner that a dual-gatetransistor is used as a transistor included in the driver circuit and aback-gate electrode of the dual-gate transistor is electricallyconnected to a capacitor line electrically connected to a capacitor;accordingly, a semiconductor device having excellent display quality canbe obtained.

In addition, the use of the semiconductor film formed in the sameformation process as the semiconductor film (specifically, the oxidesemiconductor film) of the transistor, for one electrode of thecapacitor, allows fabrication of a semiconductor device including thecapacitor whose charge capacity is increased while the aperture ratio isimproved. Further, the semiconductor device can have an excellentdisplay quality by improving the aperture ratio.

Further, oxygen vacancies and impurities such as hydrogen and nitrogenin the semiconductor film (specifically, the oxide semiconductor film)of the transistor are reduced, so that the semiconductor device of oneembodiment of the present invention has favorable electricalcharacteristics.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

Embodiment 6

The semiconductor device of one embodiment of the present invention canbe applied to any of a variety of electronic devices (including gamemachines). Examples of electronic devices include television sets (alsoreferred to as televisions or television receivers), monitors ofcomputers, cameras such as digital cameras or digital video cameras,digital photo frames, mobile phones, portable game consoles, portableinformation terminals, audio reproducing devices, game machines (e.g.,pachinko machines or slot machines), housings of game machines, and thelike. Examples of such electronic devices are illustrated in FIGS. 38Ato 38C.

FIG. 38A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. Note that thehousing 9001 is supported by four leg portions 9002. Further, a powercord 9005 for supplying power is provided for the housing 9001.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9003. Thus, the display quality of thedisplay portion 9003 can be improved.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her finger or the like, the user can carryout operation of the screen and input of information. Further, when thetable 9000 may be made to communicate with home appliances or controlthe home appliances, the table 9000 may serve as a control device whichcontrols the home appliances by operation on the screen. For example,with the use of a semiconductor device having an image sensor function,the display portion 9003 can have a touch-input function.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table 9000 can also be used as a television device. When atelevision device having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be efficiently used.

FIG. 38B illustrates a television set 9100. In the television set 9100,a display portion 9103 is incorporated in a housing 9101 and an imagecan be displayed on the display portion 9103. Note that here, thehousing 9101 is supported by a stand 9105.

The television set 9100 can be operated with an operation switch of thehousing 9101 or a separate remote controller 9110. Channels and volumecan be controlled with an operation key 9109 of the remote controller9110 so that an image displayed on the display portion 9103 can becontrolled. Further, the remote controller 9110 may be provided with adisplay portion 9107 for displaying data output from the remotecontroller 9110.

The television set 9100 illustrated in FIG. 38B is provided with areceiver. a modem, and the like. With the use of the receiver, thetelevision set 9100 can receive general TV broadcasts. Moreover, whenthe television set 9100 is connected to a communication network with orwithout wires via the modem, one-way (from a sender to a receiver) ortwo-way (between a sender and a receiver or between receivers)information communication can be performed.

The semiconductor device described in any of the above embodiments canbe used for the display portions 9103 and 9107. Thus, the displayquality of the television set can be improved.

FIG. 38C illustrates a computer, which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9203. Thus, the display quality of thecomputer can be improved.

FIGS. 39A and 39B illustrate a foldable tablet terminal. In FIG. 39A,the tablet terminal is opened, and includes a housing 9630, a displayportion 9631 a, a display portion 9631 b, a display-mode switchingbutton 9034, a power button 9035, a power-saving-mode switching button9036, a clip 9033, and an operation button 9038.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9631 a and the display portion 9631 b.Thus, the display quality of the tablet terminal can be improved.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Note that FIG. 39A shows, as an example, that half of thearea of the display portion 9631 a has only a display function, and theother half of the area has a touch panel function. However, thestructure of the display portion 9631 a is not limited to this, and allthe area of the display portion 9631 a may have a touch panel function.For example, all the area of the display portion 9631 a can displaykeyboard buttons and serve as a touch panel while the display portion9631 b can be used as a display screen.

In the display portion 9631 b, as in the display portion 9631 a, part ofthe display portion 9631 b can be a touch panel region 9632 b. When afinger, a stylus, or the like touches the place where a button 9639 forswitching to keyboard display is displayed in the touch panel, keyboardbuttons can be displayed on the display portion 9631 b.

Touch input can be performed concurrently on the touch panel regions9632 a and 9632 b.

The display-mode switching button 9034 allows switching between aportrait mode and a landscape mode, and between monochrome display andcolor display, for example. With the power-saving-mode switching button9036 for switching to power-saving mode, the luminance of display can beoptimized in accordance with the amount of external light at the timewhen the tablet terminal is in use, which is detected with an opticalsensor incorporated in the tablet terminal. The tablet terminal mayinclude another detection device such as a sensor for detectingorientation (e.g., a gyroscope or an acceleration sensor) in addition tothe optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 39A, one embodiment of the presentinvention is not limited to this example. The display portion 9631 a andthe display portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

FIG. 39B illustrates the tablet terminal folded, which includes thehousing 9630, a solar battery 9633, and a charge and discharge controlcircuit 9634. Note that FIG. 39B illustrates an example in which thecharge and discharge control circuit 9634 includes a battery 9635 and aDCDC converter 9636.

Since the tablet terminal can be foldable, the housing 9630 can beclosed when the tablet terminal is not in use. Thus, the displayportions 9631 a and 9631 b can be protected, whereby a tablet terminalwith high endurance and high reliability for long-term use can beprovided.

The tablet terminal illustrated in FIGS. 39A and 39B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar battery 9633, which is attached to the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar battery9633 can be provided on one or both surfaces of the housing 9630, sothat the battery 9635 can be charged efficiently. When a lithium ionbattery is used as the battery 9635, there is an advantage of downsizingor the like.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 39B are described with reference to a blockdiagram of FIG. 39C. The solar battery 9633, the battery 9635, the DCDCconverter 9636, a converter 9637, switches SW1, SW2, and SW3, and thedisplay portion 9631 are illustrated in FIG. 39C, and the battery 9635,the DCDC converter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 39B.

First, an example of operation in the case where power is generated bythe solar battery 9633 using external light is described. The voltage ofpower generated by the solar battery 9633 is raised or lowered by theDCDC converter 9636 so that a voltage for charging the battery 9635 isobtained. When the display portion 9631 is operated with the power fromthe solar battery 9633, the switch SW1 is turned on and the voltage ofthe power is raised or lowered by the converter 9637 to a voltage neededfor operating the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off andthe switch SW2 is turned on so that charge of the battery 9635 may beperformed.

Here, the solar battery 9633 is illustrated as an example of a powergeneration means; however, there is no particular limitation on a way ofcharging the battery 9635, and the battery 9635 may be charged withanother power generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 9635 may be charged with a non-contact power transmission modulewhich is capable of charging by transmitting and receiving power bywireless (without contact), or another charging means may be used incombination.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

Example 1

In this example, the resistances of an oxide semiconductor film and amultilayer film will be described with reference to FIGS. 40A to 40D andFIG. 41.

First, the structure of a sample is described with reference to FIGS.40A to 40D.

FIG. 40A is a top view of a sample 1, a sample 2, a sample 3, and asample 4, and FIGS. 40B to 40D are cross-sectional views taken alongdashed-and-dotted line A1-A2 in FIG. 40A. Note that the top views of thesamples 1 to 4 are the same, and the cross-sectional views thereof aredifferent because the stacked-layer structures of the cross sections aredifferent. The cross-sectional views of the sample 1, the sample 2, andthe samples 3 and 4 are illustrated in FIG. 40B, FIG. 40C, and FIG. 40D,respectively.

As for the sample 1, an insulating film 1903 is formed over a glasssubstrate 1901, an insulating film 1904 is formed over the insulatingfilm 1903, and an oxide semiconductor film 1905 is formed over theinsulating film 1904. The both ends of the oxide semiconductor film 1905are covered with a conductive film 1907 and a conductive film 1909 eachserving as an electrode, and the oxide semiconductor film 1905 and theconductive films 1907 and 1909 are covered with an insulating film 1910and an insulating film 1911. Note that an opening 1913 and an opening1915 are provided in the insulating films 1910 and 1911, and theconductive film 1907 and the conductive film 1909 are exposed throughthe opening 1913 and the opening 1915, respectively.

As for the sample 2, the insulating film 1903 is formed over the glasssubstrate 1901, the insulating film 1904 is formed over the insulatingfilm 1903, and the oxide semiconductor film 1905 is formed over theinsulating film 1904. The both ends of the oxide semiconductor film 1905are covered with the conductive films 1907 and 1909 each serving as anelectrode, and the oxide semiconductor film 1905 and the conductivefilms 1907 and 1909 are covered with the insulating film 1911. Note thatan opening 1917 and an opening 1919 are provided in the insulating film1911, and the conductive film 1907 and the conductive film 1909 areexposed through the opening 1917 and the opening 1919, respectively.

In each of the samples 3 and 4, the insulating film 1903 is formed overthe glass substrate 1901, the insulating film 1904 is formed over theinsulating film 1903, and a multilayer film 1906 is formed over theinsulating film 1904. The both ends of the multilayer film 1906 arecovered with the conductive films 1907 and 1909 each serving as anelectrode, and the multilayer film 1906 and the conductive films 1907and 1909 are covered with the insulating film 1911. Note that theopenings 1917 and 1919 are provided in the insulating film 1911, and theconductive film 1907 and the conductive film 1909 are exposed throughthe opening 1917 and the opening 1919, respectively.

As described above, the structures of the insulating films in contactwith the top surface of the oxide semiconductor film 1905 or themultilayer film 1906 are different in the samples 1 to 4. In the sample1, the oxide semiconductor film 1905 and the insulating film 1910 are incontact with each other; in the sample 2, the oxide semiconductor film1905 and the insulating film 1911 are in contact with each other; and inthe samples 3 and 4, the multilayer film 1906 and the insulating film1911 are in contact with each other.

Next, fabrication methods of the samples are described.

First, a fabrication method of the sample 1 is described.

A 400-nm-thick silicon nitride film was formed as the insulating film1903 over the glass substrate 1901 by a plasma CVD method.

Next, a 50-nm-thick silicon oxynitride film was formed as the insulatingfilm 1904 over the insulating film 1903 by a plasma CVD method.

Next, a 35-nm-thick IGZO film was formed as the oxide semiconductor film1905 over the insulating film 1904 by a sputtering method using a metaloxide target containing In, Ga, and Zn at an atomic ratio of 1:1:1.Then, etching treatment was performed on the IGZO film with a maskformed through a photolithography process, so that the oxidesemiconductor film 1905 was formed.

Next, the conductive films 1907 and 1909 were formed over the insulatingfilm 1904 and the oxide semiconductor film 1905 in such a manner that a50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a100-nm-thick titanium film were stacked in this order by a sputteringmethod, and were then subjected to etching treatment with a mask formedthrough a photolithography process.

Next, a 450-nm-thick silicon oxynitride film was formed as theinsulating film 1910 over the insulating film 1904, the oxidesemiconductor film 1905, the conductive film 1907, and the conductivefilm 1909 by a plasma CVD method, and then heat treatment was performedat 350° C. under a mixed atmosphere of nitrogen and oxygen for one hour.

Next, a 50-nm-thick silicon nitride film was formed as the insulatingfilm 1911 over the insulating film 1910 by a plasma CVD method.

Next, a mask is formed over the insulating film 1911 through aphotolithography process and then etching treatment was performed on theinsulating film 1911, so that the openings 1913 and 1915 were formed inthe insulating films 1910 and 1911.

Through the above process, the sample 1 was fabricated.

Next, a fabrication method of the sample 2 is described.

Next, a 450-nm-thick silicon oxynitride film was formed as theinsulating film 1910 over the insulating film 1904, the oxidesemiconductor film 1905, the conductive film 1907, and the conductivefilm 1909 of the sample 1 by a plasma CVD method, and then heattreatment was performed at 350° C. under a mixed atmosphere of nitrogenand oxygen for one hour. After that, the insulating film 1910 wasremoved.

Next, a 50-nm-thick silicon nitride film was formed as the insulatingfilm 1911 over the insulating film 1904, the oxide semiconductor film1905, the conductive film 1907, and the conductive film 1909 by a plasmaCVD method.

Next, a mask is formed over the insulating film 1911 through aphotolithography process and then etching treatment was performed on theinsulating film 1911, so that the openings 1917 and 1919 were formed inthe insulating film 1911.

Through the above process, the sample 2 was fabricated.

Next, a fabrication method of the sample 3 is described.

As for the sample 3, the multilayer film 1906 was used instead of theoxide semiconductor film 1905 of the sample 2. The multilayer film 1906was formed over the insulating film 1904 in such a manner that a10-nm-thick IGZO film with a metal oxide target containing In, Ga, andZn at an atomic ratio of 1:3:2, a 10-nm-thick IGZO film with a metaloxide target containing In, Ga, and Zn at an atomic ratio of 1:1:1, andthen a 10-nm-thick IGZO film with a metal oxide target containing In,Ga, and Zn at an atomic ratio of 1:3:2 were successively formed by asputtering method. Then, etching treatment was performed on the stackedIGZO films with a mask formed through a photolithography process, sothat the multilayer film 1906 was formed.

Through the above process, the sample 3 was fabricated.

Next, a fabrication method of the sample 4 is described.

As for the sample 4, the multilayer film 1906 was used instead of theoxide semiconductor film 1905 of the sample 2. The multilayer film 1906was formed over the insulating film 1904 in such a manner that a20-nm-thick IGZO film with a metal oxide target containing In, Ga, andZn at an atomic ratio of 1:3:2, a 15-nm-thick IGZO film with a metaloxide target containing In, Ga, and Zn at an atomic ratio of 1:1:1, andthen a 10-nm-thick IGZO film using a metal oxide target containing In,Ga, and Zn at an atomic ratio of 1:3:2 were successively formed by asputtering method. Then, etching treatment was performed on the stackedIGZO films with a mask formed through a photolithography process, sothat the multilayer film 1906 was formed.

Through the above process, the sample 4 was fabricated.

Next, the sheet resistance of the oxide semiconductor film 1905 providedin each of the samples 1 and 2 and the sheet resistance of themultilayer film 1906 provided in each of the samples 3 and 4 weremeasured. In the sample 1, a probe is made contact with the openings1913 and 1915 to measure the sheet resistance of the oxide semiconductorfilm 1905. In each of the samples 2 to 4, a probe is made contact withthe openings 1917 and 1919 to measure the sheet resistance of the oxidesemiconductor film 1905 or the multilayer film 1906. Note that in theoxide semiconductor film 1905 in each of the samples 1 and 2 and themultilayer film 1906 in each of the samples 3 and 4, the widths of theconductive films 1907 and 1909 facing each other were each 1 mm and thedistance between the conductive films 1907 and 1909 was 10 μm. Further,in each of the samples 1 to 4, the potential of the conductive film 1907was a ground potential, and 1 V was applied to the conductive film 1909.

FIG. 41 shows the sheet resistance of the samples 1 to 4.

The sheet resistance of the sample 1 was about 1×10¹¹ Ω/sq. The sheetresistance of the sample 2 was about 2620 Ω/sq. The sheet resistance ofthe sample 3 was about 4410 Ω/sq. The sheet resistance of the sample 4was about 2930 Ω/sq.

In the above manner, the oxide semiconductor films 1905 and themultilayer films 1906 have different values of sheet resistance becausethe insulating films in contact with the oxide semiconductor film 1905and the insulating films in contact with the multilayer film 1906 weredifferent.

Note that when the above sheet resistances of the samples 1 to 4 wereconverted into resistivity, the resistivities of the sample 1, thesample 2, the sample 3, and the sample 4 were 3.9×10⁵ Ωcm, 9.3×10⁻³ Ωcm,1.3×10⁻² Ωcm, and 1.3×10⁻² Ωcm, respectively.

In the sample 1, the silicon oxynitride film used as the insulating film1910 was formed in contact with the top surface of the oxidesemiconductor film 1905 and apart from the silicon nitride film used asthe insulating film 1911. On the other hand, the silicon nitride filmused as the insulating film 1911 was formed in contact with the topsurface of the oxide semiconductor film 1905 in the sample 2 and wasformed in contact with the top surface of the multilayer film 1906 ineach of the samples 3 and 4. When the oxide semiconductor film 1905 orthe multilayer film 1906 is thus provided in contact with the siliconnitride film used as the insulating film 1911, defects, typically oxygenvacancies are generated in the oxide semiconductor film 1905 or themultilayer film 1906, and hydrogen contained in the silicon nitride filmis transferred to or diffused into the oxide semiconductor film 1905 orthe multilayer film 1906. Accordingly, the conductivity of the oxidesemiconductor film 1905 or the multilayer film 1906 is improved.

For example, in the case where an oxide semiconductor film is used for achannel formation region of a transistor, it is preferable to employ astructure in which a silicon oxynitride film is provided in contact withthe oxide semiconductor film as shown in the sample 1. Further, as alight-transmitting conductive film used for an electrode of a capacitor,it is preferable to employ a structure in which a silicon nitride filmis provided in contact with an oxide semiconductor film or a multilayerfilm as shown in the samples 2 to 4. With such a structure, even when anoxide semiconductor film or a multilayer film which is used for achannel formation region of a transistor and an oxide semiconductor filmor a multilayer film which is used for an electrode of a capacitor areformed through the same process, the resistivity of the oxidesemiconductor film and the resistivity of the multilayer film can bemade different from each other.

Next, the sheet resistance values of the samples 2 and 3 which werepreserved under a high-temperature high-humidity environment weremeasured. The conditions of the samples used here are described below.Note that here, the conditions are partly different from those of thesamples 2 and 3. Therefore, samples which have the same structure as thesamples 2 and 3 and which were formed under the different formationconditions are referred to as a sample 2 a and a sample 3 a.

First, a fabrication method of the sample 2 a is described.

The insulating film 1903 and the insulating film 1904 were formed overthe glass substrate 1901.

Next, a 35-nm-thick IGZO film was formed as the oxide semiconductor film1905 over the insulating film 1904 by a sputtering method using a metaloxide target containing In, Ga, and Zn at an atomic ratio of 1:1:1.Then, etching treatment was performed on the IGZO film with a maskformed through a photolithography process and then heat treatment wasperformed at 350° C. or 450° C., so that the oxide semiconductor film1905 was formed.

Next, the conductive film 1907 and the conductive film 1909 were formedover the insulating film 1904 and the oxide semiconductor film 1905 insuch a manner that a 50-nm-thick titanium film and a 400-nm-thick copperfilm were stacked in this order by a sputtering method, and were thensubjected to etching treatment with a mask formed through aphotolithography process.

Next, a 450-nm-thick silicon oxynitride film was formed as theinsulating film 1910 over the insulating film 1904, the oxidesemiconductor film 1905, the conductive film 1907, and the conductivefilm 1909 by a plasma CVD method, and then heat treatment was performedat 350° C. under a mixed atmosphere of nitrogen and oxygen for one hour.

Next, a 50-nm-thick silicon nitride film was formed as the insulatingfilm 1911 over the insulating film 1904, the oxide semiconductor film1905, the conductive film 1907, and the conductive film 1909 by a plasmaCVD method. Note that the film formation temperature of the siliconnitride film was 220° C. or 350° C.

Next, a mask is formed over the insulating film 1911 through aphotolithography process and then etching treatment was performed on theinsulating film 1911, so that the opening 1917 and 1919 were formed inthe insulating films 1910 and 1911.

Through the above process, the sample 2 a was fabricated.

Next, a fabrication method of the sample 3 a is described.

As for the sample 3 a, the multilayer film 1906 was used instead of theoxide semiconductor film 1905 of the sample 2 a. The multilayer film1906 was formed over the insulating film 1904 in such a manner that a10-nm-thick IGZO film with a metal oxide target containing In, Ga, andZn at an atomic ratio of 1:1:1, and a 10-nm-thick IGZO film with a metaloxide target containing In, Ga, and Zn at an atomic ratio of 1:3:2 weresuccessively formed by a sputtering method. Then, etching treatment wasperformed on the stacked IGZO films with a mask formed through aphotolithography process and then heat treatment was performed at 350°C. or 450° C., so that the multilayer film 1906 was formed.

Through the above process, the sample 3 a was fabricated.

Next, the sheet resistance of the oxide semiconductor film 1905 providedin the sample 2 a and the sheet resistance of the multilayer film 1906provided in the sample 3 a were measured. In each of the samples 2 a and3 a, a probe is made contact with the openings 1917 and 1919 to measurethe sheet resistance of the oxide semiconductor film 1905 or themultilayer film 1906. Note that in the oxide semiconductor film 1905 inthe sample 2 a and the multilayer film 1906 in the sample 3 a, thewidths of the conductive films 1907 and 1909 facing each other were each1.5 mm and the distance between the conductive films 1907 and 1909 was10 μm. Further, in each of the samples 2 a and 3 a, the potential of theconductive film 1907 was a ground potential, and 1 V was applied to theconductive film 1909. The sheet resistance values of the samples 2 a and3 a were measured after the samples 2 a and 3 a were preserved at 60° C.under an atmosphere with a humidity of 95% for 60 hours and 130 hours.

FIG. 45 shows the sheet resistance values of the samples 2 a and 3 a.Note that in FIG. 45, the film formation temperature of the siliconnitride film formed as the insulating film 1911 in each sample is 220°C. (a solid line) or 350° C. (a dashed line). In addition, black circleand triangle indicate the samples each subjected to heat treatment at350° C. after the formation of the oxide semiconductor film 1905 or themultilayer film 1906, and white circle and triangle indicate the sampleseach subjected to heat treatment at 450° C. after the formation of theoxide semiconductor film 1905 or the multilayer film 1906. The black andwhite triangles indicate the samples each including the oxidesemiconductor film 1905, i.e. the sample 2 a, and the black and whitecircles indicate the samples each including the multilayer film 1906,i.e. the sample 3 a.

FIG. 45 shows that the samples 2 a and 3 a had low sheet resistancevalues and satisfied a preferable sheet resistance value for anelectrode of a capacitor, which is 0.2 M/sq., and that the amount ofchange over time in the sheet resistance values of the samples 2 a and 3a was small. As described above, the amount of change in the sheetresistance value of the oxide semiconductor film or the multilayer filmin contact with the silicon nitride film is small under ahigh-temperature high-humidity environment; therefore, the oxidesemiconductor film or the multilayer film can be used as alight-transmitting conductive film which is used for an electrode of acapacitor.

Next, the sheet resistance values of the samples 2 a and 3 a when thesubstrate temperature was 25° C., 60° C., or 150° C. were measured, andthe measurement results are shown in FIG. 46. Note that here, as each ofthe samples 2 a and 3 a, a sample which includes the silicon nitridefilm formed as the insulating film 1911 at 220° C. and which wassubjected to heat treatment at 350° C. after the formation of themultilayer film 1906 was used.

FIG. 46 shows that the sheet resistance value of the multilayer film1906 was not changed even when the substrate temperature was raised. Inother words, the oxide semiconductor film or the multilayer film incontact with the silicon nitride film is a degenerated semiconductor.The amount of change in the sheet resistance value of the oxidesemiconductor film or the multilayer film in contact with the siliconnitride film was small even when the substrate temperature was changed;therefore, the oxide semiconductor film or the multilayer film can beused as a light-transmitting conductive film which is used for anelectrode of a capacitor.

Note that the structure described in this example can be used asappropriate in combination with any of the structures in the otherembodiments and examples.

Example 2

In this example, analysis of impurities in an oxide semiconductor filmand an insulating film formed over the oxide semiconductor film will bedescribed with reference to FIGS. 42A and 42B.

In this example, two kinds of samples (hereinafter a sample 5 and asample 6) were formed as samples for impurity analysis.

First, a fabrication method of the sample 5 is described below.

As for the sample 5, an IGZO film was formed over a glass substrate anda silicon nitride film was formed thereover. After that, heat treatmentat 450° C. under a nitrogen atmosphere for one hour and then heattreatment at 450° C. under a mixed gas atmosphere of nitrogen and oxygen(the proportion of the nitrogen was 80%, and the proportion of theoxygen was 20%) for one hour were successively performed.

Note that as for the IGZO film, a 100-nm-thick IGZO film was formed by asputtering method using a metal oxide target containing In, Ga, and Znat an atomic ratio of 1:1:1 under the following conditions: the Ar gasflow rate was 100 sccm and the O₂ gas flow rate was 100 sccm (theproportion of the O₂ gas was 50%); the pressure was 0.6 Pa; the filmformation power was 5000 W; and the substrate temperature was 170° C.

In addition, as for the silicon nitride film, a 100-nm-thick siliconnitride film was formed by a PE-CVD method under the followingconditions: the SiH₄ gas flow rate was 50 sccm, the N₂ gas flow rate was5000 sccm, and the NH₃ gas flow rate was 100 sccm; the pressure was 100Pa; the film formation power was 1000 W; and the substrate temperaturewas 220° C.

Next, a fabrication method of the sample 6 is described below.

An IGZO film was formed over a glass substrate and a silicon oxynitridefilm and a silicon nitride film were stacked thereover. After that, heattreatment at 450° C. under a nitrogen atmosphere for one hour and thenheat treatment at 450° C. under a mixed gas atmosphere of nitrogen andoxygen (the proportion of the nitrogen was 80%, and the proportion ofthe oxygen was 20%) for one hour were successively performed.

Note that the film formation conditions of the IGZO film and the siliconnitride film were similar to those of the sample 5. In addition, as forthe silicon oxynitride film, a 50-nm-thick silicon oxynitride film wasformed by a PE-CVD method under the following conditions: the SiH₄ gasflow rate was 30 sccm and the N₂O gas flow rate was 4000 sccm; thepressure was 40 Pa; the film formation power was 150 W; and thesubstrate temperature was 220° C. After that, a 400-nm-thick siliconoxynitride film was formed by a PE-CVD method under the followingconditions: the SiH₄ gas flow rate was 160 sccm and the N₂O gas flowrate was 4000 sccm; the pressure was 200 Pa; the film formation powerwas 1500 W; and the substrate temperature was 220° C.

FIGS. 42A and 42B show the results of the impurity analysis of thesamples 5 and 6.

Note that the impurity analysis was performed in the direction shown bythe arrow in each of FIGS. 42A and 42B by secondary ion massspectrometry (SIMS). That is, the measurement was performed from theglass substrate side.

FIG. 42A shows the concentration profile of hydrogen (H) which wasobtained by measurement of the sample 5. FIG. 42B shows theconcentration profile of hydrogen (H) which was obtained by measurementof the sample 6.

FIG. 42A shows that the concentration of hydrogen (H) in the IGZO filmwas 1.0×10²⁰ atoms/cm³ and that the concentration of hydrogen (H) in thesilicon nitride film was 1.0×10²³ atoms/cm³. In addition, FIG. 42B showsthat the concentration of hydrogen (H) in the IGZO film was 5.0×10¹⁹atoms/cm³ and that the concentration of hydrogen (H) in the siliconoxynitride film was 3.0×10²¹ atoms/cm³.

It is known that it is difficult to obtain accurate data in theproximity of a surface of a sample or in the proximity of an interfacebetween stacked films formed using different materials by the SIMSanalysis in measurement principle. Thus, in the case where distributionsof the concentrations of hydrogen (H) in the film in the thicknessdirection are analyzed by SIMS, an average value in a region where thefilm is provided, the value is not greatly changed, and an almostconstant level of strength can be obtained is employed as theconcentrations of hydrogen (H).

A difference between the IGZO films in the concentration of hydrogen (H)was found in this manner by changing the structure of the insulatingfilm in contact with the IGZO film.

For example, in the case where any of the above IGZO films is formed ina channel formation region of a transistor, it is preferable to employ astructure in which a silicon oxynitride film is provided in contact withthe IGZO film as shown in the sample 6. As a light-transmittingconductive film used for an electrode of a capacitor, it is preferableto employ a structure in which a silicon nitride film is provided incontact with the IGZO film as shown in the sample 5. With such astructure, even when an IGZO film which is used for a channel formationregion of a transistor and an IGZO film which is used for an electrodeof a capacitor are formed through the same process, the hydrogenconcentrations of the IGZO films can be made different from each other.

Example 3

In this example, the amounts of defects in an oxide semiconductor filmand a multilayer film will be described with reference to FIGS. 43A to43C and FIG. 44.

First, the structure of a sample is described.

A sample 7 includes a 35-nm-thick oxide semiconductor film formed over aquartz substrate and a 100-nm-thick nitride insulating film formed overthe oxide semiconductor film.

A sample 8 and a sample 9 each include a 30-nm-thick multilayer filmformed over a quartz substrate and a 100-nm-thick nitride insulatingfilm formed over the multilayer film. Note that in the multilayer filmof the sample 8, a 10-nm-thick first oxide film, a 10-nm-thick oxidesemiconductor film, and a 10-nm-thick second oxide film are stacked inthis order. In the multilayer film of the sample 9, a 20-nm-thick firstoxide film, a 15-nm-thick oxide semiconductor film, and a 10-nm-thicksecond oxide film are stacked in this order. The samples 8 and 9 aredifferent from the sample 7 in that the multilayer film is includedinstead of the oxide semiconductor film.

A sample 10 includes a 100-nm-thick oxide semiconductor film formed overa quartz substrate, a 250-nm-thick oxide insulating film formed over theoxide semiconductor film, and a 100-nm-thick nitride insulating filmformed over the oxide insulating film. The sample 10 is different fromthe samples 7 to 9 in that the oxide semiconductor film is not incontact with the nitride insulating film but in contact with the oxideinsulating film.

Next, fabrication methods of the samples are described.

First, a fabrication method of the sample 7 is described.

A 35-nm-thick IGZO film was formed as the oxide semiconductor film overthe quartz substrate. As for the IGZO film, the 35-nm-thick IGZO filmwas formed by a sputtering method using a metal oxide target containingIn, Ga, and Zn at an atomic ratio of 1:1:1 under the followingconditions: the Ar gas flow rate was 100 sccm and the O₂ gas flow ratewas 100 sccm (the proportion of the O₂ gas was 50%); the pressure was0.6 Pa; the film formation power was 5000 W; and the substratetemperature was 170° C.

Next, as first heat treatment, heat treatment at 450° C. under anitrogen atmosphere for one hour and then heat treatment at 450° C.under a mixed gas atmosphere of nitrogen and oxygen (the proportion ofthe nitrogen was 80%, and the proportion of the oxygen was 20%) for onehour were successively performed.

Next, a 100-nm-thick silicon nitride film was formed as the nitrideinsulating film over the oxide semiconductor film. As for the siliconnitride film, the 100-nm-thick silicon nitride film was formed by aPE-CVD method under the following conditions: the SiH₄ gas flow rate was50 sccm, the N₂ gas flow rate was 5000 sccm, and the NH₃ gas flow ratewas 100 sccm; the pressure was 100 Pa; the film formation power was 1000W; and the substrate temperature was 350° C.

Next, as second heat treatment, heat treatment was performed at 250° C.under a nitrogen atmosphere for one hour.

Through the above process, the sample 7 was fabricated.

Next, a fabrication method of the sample 8 is described.

As for the sample 8, the multilayer film was formed instead of the oxidesemiconductor film of the sample 7. As for the multilayer film, the10-nm-thick first oxide film was formed by a sputtering method using ametal oxide target containing In, Ga, and Zn at an atomic ratio of 1:3:2under the following conditions: the Ar gas flow rate was 180 sccm andthe O₂ gas flow rate was 20 sccm (the proportion of the O₂ gas was 10%);the pressure was 0.6 Pa; the film formation power was 5000 W; and thesubstrate temperature was 25° C. Then, the 10-nm-thick oxidesemiconductor film was formed by a sputtering method using a metal oxidetarget containing In, Ga, and Zn at an atomic ratio of 1:1:1 under thefollowing conditions: the Ar gas flow rate was 100 sccm and the O₂ gasflow rate was 100 sccm (the proportion of the O₂ gas was 50%); thepressure was 0.6 Pa; the film formation power was 5000 W; and thesubstrate temperature was 170° C. Then, the 10-nm-thick second oxidefilm was formed by a sputtering method using a metal oxide targetcontaining In, Ga, and Zn at an atomic ratio of 1:3:2 under thefollowing conditions: the Ar gas flow rate was 180 sccm and the O₂ gasflow rate was 20 sccm (the proportion of the O₂ gas was 10%); thepressure was 0.6 Pa; the film formation power was 5000 W; and thesubstrate temperature was 25° C.

Other steps are similar to those of the sample 7. Through the aboveprocess, the sample 8 was fabricated.

Next, a fabrication method of the sample 9 is described.

As for the sample 9, the multilayer film was formed instead of the oxidesemiconductor film of the sample 7. As for the multilayer film, the20-nm-thick first oxide film was formed over the quartz substrate underthe same conditions as the first oxide film of the sample 8. Then, the15-nm-thick oxide semiconductor film was formed by a sputtering methodunder the same conditions as the oxide semiconductor film of the sample8. Then, the 10-nm-thick second oxide film was formed under the sameconditions as the second oxide film of the sample 8.

Other steps are similar to those of the sample 7. Through the aboveprocess, the sample 9 was fabricated.

Next, a fabrication method of the sample 10 is described.

As for the sample 10, the 100-nm-thick oxide semiconductor film wasformed over the quartz substrate under the same conditions as the sample7.

Next, first heat treatment was performed under conditions similar tothose of the sample 7.

Next, a 50-nm-thick first silicon oxynitride film and a 200-nm-thicksecond silicon oxynitride film were stacked over the oxide semiconductorfilm as the oxide insulating film. Here, the 50-nm-thick first siliconoxynitride film was formed by a PE-CVD method under the followingconditions: the SiH₄ gas flow rate was 30 sccm and the N₂O gas flow ratewas 4000 sccm; the pressure was 40 Pa; the film formation power was 150W; and the substrate temperature was 220° C. After that, the200-nm-thick second silicon oxynitride film was formed by a PE-CVDmethod under the following conditions: the SiH₄ gas flow rate was 160sccm and the N₂O gas flow rate was 4000 sccm; the pressure was 200 Pa;the film formation power was 1500 W; and the substrate temperature was220° C. Note that the second silicon oxynitride film is a filmcontaining oxygen at a higher proportion than oxygen in thestoichiometric composition.

Next, a 100-nm-thick silicon nitride film was formed over the oxideinsulating film under the same conditions as the sample 7.

Next, second heat treatment was performed under conditions similar tothose of the sample 7.

Through the above process, the sample 10 was fabricated.

Next, the samples 7 to 10 were measured by ESR. In the ESR measurementperformed at a predetermined temperature, a value of a magnetic field(H₀) where a microwave is absorbed is used for an equation g=hn/bH₀, sothat a parameter of a g-factor can be obtained. Note that the frequencyof the microwave is denoted by ν, and the Planck constant and the Bohrmagneton are denoted by, respectively, h and β which are both constants.

Here, the ESR measurement was performed under the conditions as follows.The measurement temperature was room temperature (25° C.), thehigh-frequency power (power of microwaves) of 8.92 GHz was 20 mW, andthe direction of a magnetic field was parallel to a surface of eachsample.

FIG. 43A shows a first derivative curve obtained by ESR measurement ofthe oxide semiconductor film in the sample 7; and FIGS. 43B and 43C showfirst derivative curves obtained by ESR measurement of the multilayerfilms in the samples 8 and 9. FIG. 43A shows the measurement result ofthe sample 7, FIG. 43B shows the measurement result of the sample 8, andFIG. 43C shows the measurement result of the sample 9.

FIG. 44 shows a first derivative curve obtained by ESR measurement ofthe oxide semiconductor film in the sample 10.

In FIGS. 43A to 43C, the sample 7 has signal symmetry due to a defect inthe oxide semiconductor film when a g-factor is 1.93. The samples 8 and9 each have signal symmetry due to a defect in the multilayer film whena g-factor is 1.95. As for the sample 7, the spin density when ag-factor was 1.93 was 2.5×10¹⁹ spins/cm³, in the sample 8, the totalspin densities when g-factors were 1.93 and 1.95 were 1.6×10¹⁹spins/cm³, and in the sample 9, the total spin densities when g-factorswere 1.93 and 1.95 were 2.3×10¹⁹ spins/cm³. That is, it is found thatthe oxide semiconductor film and the multilayer film include defects.Note that an oxygen vacancy is an example of the defect in the oxidesemiconductor film and the multilayer film.

Although, in FIG. 44, the thickness of the oxide semiconductor film ofthe sample 10 is thicker than that of the sample 7 and those of themultilayer films of the samples 8 and 9, signal symmetry due to a defectwas not detected, i.e. the number of defects was less than or equal tothe lower limit of detection (here, the lower limit of detection was3.7×10¹⁶ spins/cm³). Accordingly, it is found that the number of defectsin the oxide semiconductor film cannot be detected.

It is found that when a nitride insulating film, here the siliconnitride film formed by a PE-CVD method is in contact with an oxidesemiconductor film or a multilayer film, defects, typically oxygenvacancies are generated in the oxide semiconductor film or themultilayer film. On the other hand, when an oxide insulating film, herethe silicon oxynitride film is provided over an oxide semiconductorfilm, excess oxygen contained in the silicon oxynitride film, i.e.oxygen contained at a higher proportion than oxygen in thestoichiometric composition is diffused into the oxide semiconductor filmand thus the number of defects in the oxide semiconductor film is notincreased.

As described above, as shown in the samples 7 to 9, the oxidesemiconductor film or the multilayer film which is in contact with thenitride insulating film has a number of defects, typically oxygenvacancies, and has a high conductivity and therefore can be used as anelectrode of a capacitor. On the other hand, as shown in the sample 10,an oxide semiconductor film which is in contact with the oxideinsulating film has a small number of oxygen vacancies and lowconductivity and therefore can be used as a channel formation region ofa transistor.

Here, the cause of a reduction in resistivity of the oxide semiconductorfilm or the multilayer film which is in contact with the nitrideinsulating film is described below.

<Energy and Stability Between Existing Modes of Hydrogen (H)>

First, the energy difference and stability in a mode of H which existsin an oxide semiconductor film is described with calculated results.Here, InGaZnO₄ was used as the oxide semiconductor film.

The structure used for the calculation is based on a 84-atom bulk modelin which twice the number of a hexagonal unit cell of the InGaZnO₄ isarranged along the a-axis and b-axis.

As the bulk model, a model in which one O atom bonded to three In atomsand one Zn atom is substituted with a H atom was prepared (see FIG.47A). FIG. 47B shows a diagram in which the a-b plane of the InO layerin FIG. 47A is viewed from the c-axis direction. A region from which oneO atom bonded to three In atoms and one Zn atom is removed is shown asan oxygen vacancy Vo, which is shown in a dashed line in FIGS. 47A and47B. In addition, a H atom in the oxygen vacancy Vo is expressed as VoH.

In the bulk model, one O atom bonded to three In atoms and one Zn atomis removed, whereby an oxygen vacancy Vo is formed. A model in which, inthe vicinity of the oxygen vacancy Vo, a H atom is bonded to one O atomto which one Ga atom and two Zn atoms are bonded on the a-b plane wasprepared (see FIG. 47C). FIG. 47D shows a diagram in which the a-b planeof the InO layer in FIG. 47C is viewed from the c-axis direction. InFIGS. 47C and 47D, an oxygen vacancy Vo is shown in a dashed line. Amodel in which an oxygen vacancy Vo is formed and, in the vicinity ofthe oxygen vacancy Vo, a H atom is bonded to one O atom to which one Gaatom and two Zn atoms are bonded on the a-b plane is expressed as Vo+H.

Optimization calculation was performed on the above two models with afixed lattice constant to calculate the total energy. Note that as thevalue of the total energy is smaller, the structure becomes more stable.

In the calculation, first principles calculation software VASP (TheVienna Ab initio Simulation Package) was used. The calculationconditions are shown in Table 1.

TABLE 1 Software VASP Pseudopotential PAW Functional CGA/PBE Cut-offenergy 500 eV K-point 4 × 4 × 1

As pseudopotential calculation of electronic states, a potentialgenerated by a projector augmented wave (PAW) method was used, and as afunctional, generalized-gradient-approximation/Perdew-Burke-Ernzerhof(GGA/PBE) was used.

In addition, the total energy of the two models which were obtained bythe calculations is shown in Table 2.

TABLE 2 Model Total Energy VoH −456.084 eV Vo + H −455.304 eV

According to Table 2, the total energy of VoH is lower than that of Vo+Hby 0.78 eV. Thus, VoH is more stable than Vo+H. Accordingly, when a Hatom comes close to an oxygen vacancy (Vo), the H atom might be easilytrapped in the oxygen vacancy (Vo) than bonding with an O atom.

<Thermodynamic State of VoH>

Next, the formation energy and the charge state of VoH which isgenerated by a H atom trapped in an oxygen vacancy (Vo) is describedwith calculated results. The formation energy of VoH is differentdepending on the charge state and also depends on the Fermi energy.Thus, the stable charge state of VoH is different depending on the Fermienergy. Here, (VoH)⁺ denotes a state in which one electron is dischargedby VoH, (VoH)⁻ denotes a state in which one electron is trapped by VoH,and (VoH)⁰ denotes a state in which an electron is not transferred. Theformation energies of (VoH)⁺, (VoH)⁻, and (VoH)⁰ were calculated.

In the calculation, the first principles calculation software VASP wasused. The calculation conditions are shown in Table 3.

TABLE 3 Software VASP Pseudopotential PAW Functional HSE06 Cut-offenergy 800 eV Number of k-point sampling 2 × 2 × 1 (opt.) 4 × 4 × 1(single) Spin polarization setup Shielding parameter 0.2 Fraction of thenonolcal 0.25 Fock-exchange Number of atoms 84

As pseudopotential calculation of electronic states, a potentialgenerated by a projector augmented wave (PAW) method was used, and as afunctional, Heyd-Scuseria-Ernzerhof (HSE) DFT hybrid factor (HSE06) wasused.

Note that the formation energy of an oxygen vacancy was calculated asfollows: a dilute limit of the concentration of oxygen vacancies wasassumed, and excessive expansion of electrons and holes to theconduction band and the valence band was corrected. In addition, shiftof the valence band due to the defect structure was corrected using theaverage electrostatic potential with the top of the valence band of acomplete crystal serving as the origin of energy.

FIG. 48A shows the formation energies of (VoH)⁺, (VoH)⁻, and (VoH)⁰. Thehorizontal axis represents the Fermi level, and the vertical axisrepresents the formation energy. The solid line represents the formationenergy of (VoH)⁺, the dashed-dotted line represents the formation energyof (VoH)⁰, and the dashed line represents the formation energy of(VoH)⁻. In addition, the transition level of the VoH charge from (VoH)⁺to (VoH)⁻ through (VoH)⁰ is represented by E (+/−).

FIG. 48B shows a thermodynamic transition level of VoH. From thecalculation result, the energy gap of InGaZnO₄ was 2.739 eV. Inaddition, when the energy of the valence band is 0 eV, the transferlevel (c (+/−)) is 2.62 eV, which exists just under the conduction band.This shows that InGaZnO₄ is n-type by trapping a H atom in an oxygenvacancy Vo.

When an oxide semiconductor film is exposed to plasma, the oxidesemiconductor film is damaged and defects, typically oxygen vacanciesare generated in the oxide semiconductor film. In addition, when anitride insulating film is in contact with an oxide semiconductor film,hydrogen contained in the nitride insulating film is transferred to theoxide semiconductor film. As a result, VoH is formed in an oxidesemiconductor film by entry of hydrogen into an oxygen vacancy in theoxide semiconductor film, so that the oxide semiconductor film becomesn-type film and the resistivity thereof is reduced. As described above,the oxide semiconductor film in contact with the nitride insulating filmcan be used as an electrode of a capacitor.

This application is based on Japanese Patent Application serial No.2012-178907 filed with the Japan Patent Office on Aug. 10, 2012 andJapanese Patent Application serial No. 2013-053942 filed with the JapanPatent Office on Mar. 15, 2013, the entire contents of which are herebyincorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a gate electrode; anoxide semiconductor film; and a gate insulating film between the gateelectrode and the oxide semiconductor film, wherein the oxidesemiconductor film includes hydrogen and oxygen vacancy, and wherein theoxygen vacancy is filled with the hydrogen.
 3. The semiconductor deviceaccording to claim 2, further comprising: a pair of electrodes over andin contact with the oxide semiconductor film; a first insulating filmover the pair of electrodes; a second insulating film over the firstinsulating film; and a second gate electrode over the second insulatingfilm.
 4. The semiconductor device according to claim 2, furthercomprising: a pair of electrodes over and in contact with the oxidesemiconductor film; an insulating film over the pair of electrodes; asecond gate electrode over the insulating film; and a capacitor line,wherein the second gate electrode is electrically connected to thecapacitor line.
 5. A semiconductor device comprising: a driver circuitwhich includes a first transistor including a first oxide semiconductorfilm and a gate electrode; a pixel which includes a capacitor includinga dielectric film between a pair of electrodes; and a capacitor lineelectrically connected to the gate electrode and one of the pair ofelectrodes, wherein the first oxide semiconductor film includes hydrogenand oxygen vacancy, and wherein the oxygen vacancy is filled with thehydrogen.
 6. The semiconductor device according to claim 5, wherein thegate electrode is over the first oxide semiconductor film, and whereinthe first transistor includes a second gate electrode below the firstoxide semiconductor film.
 7. The semiconductor device according to claim5, wherein the pixel includes a second transistor including a secondoxide semiconductor film, wherein the capacitor comprises a third oxidesemiconductor film on the same surface as the second oxide semiconductorfilm, and wherein the third oxide semiconductor film is in contact withthe capacitor line.
 8. The semiconductor device according to claim 5,wherein the pixel includes a second transistor including a second oxidesemiconductor film, and wherein the capacitor line extends in adirection parallel to a signal line which is electrically connected to asource electrode and a drain electrode of the second transistor and isprovided on the same surface as the source electrode or the drainelectrode of the second transistor.
 9. A semiconductor devicecomprising: a driver circuit which includes a first transistor includinga first semiconductor film; a pixel which includes a second transistorincluding a second semiconductor film; a capacitor which includes adielectric film between a pair of electrodes and a pixel electrodeelectrically connected to the second transistor, which are in the pixel;and a capacitor line electrically connected to one of the pair ofelectrodes, wherein the first transistor includes a first gate electrodebelow the first semiconductor film and a second gate electrode above thefirst semiconductor film, wherein the second gate electrode iselectrically connected to the capacitor line, wherein an insulating filmwhich has a stacked-layer structure of an oxide insulating film and anitride insulating film is at least over the second semiconductor film,wherein the capacitor comprises a third semiconductor film on the samesurface as the second semiconductor film, and the third semiconductorfilm serves as the one of the pair of electrodes, wherein the pixelelectrode serves as the other of the pair of electrodes, wherein thedielectric film is the nitride insulating film, wherein each of thesecond semiconductor film and the third semiconductor film has alight-transmitting property and includes an oxide semiconductor, whereineach of the second semiconductor film and the third semiconductor filmincludes hydrogen and oxygen vacancy, and wherein the oxygen vacancy isfilled with the hydrogen.
 10. The semiconductor device according toclaim 9, wherein the capacitor line and the third semiconductor film arein contact with each other.
 11. The semiconductor device according toclaim 9, wherein the capacitor line extends in a direction parallel to asignal line which is electrically connected to a source electrode and adrain electrode of the second transistor and is provided on the samesurface as the source electrode and the drain electrode of the secondtransistor.
 12. The semiconductor device according to claim 9, whereinthe second gate electrode is a conductive film formed using the samematerial as the pixel electrode.
 13. The semiconductor device accordingto claim 9, wherein an organic insulating film is over a region of theinsulating film other than regions overlapping with the firsttransistor, the second transistor, and the capacitor.
 14. Thesemiconductor device according to claim 9, wherein a conductive filmformed using the same material as a source electrode or a drainelectrode of the second transistor is over an end portion of the thirdsemiconductor film.
 15. The semiconductor device according to claim 9,wherein the first semiconductor film includes an oxide semiconductor,wherein the first semiconductor film includes hydrogen and oxygenvacancy, and wherein the oxygen vacancy of the first semiconductor filmis filled with the hydrogen of the first semiconductor film.
 16. Thesemiconductor device according to claim 9, wherein the thirdsemiconductor film includes a region having a higher conductivity thanthe second semiconductor film.
 17. The semiconductor device according toclaim 16, wherein the third semiconductor film includes one or moreselected from boron, nitrogen, fluorine, aluminum, phosphorus, arsenic,indium, tin, antimony, and a rare gas element.